Optical fiber communication system using optical phase conjugation as well as apparatus applicable to the system and method of producing the same

ABSTRACT

An optical fiber communication system according to the present invention has, for example, first and second phase conjugators. The first phase conjugator converts a signal beam from a first optical fiber into a first phase conjugate beam. The first phase conjugate beam is supplied to the second phase conjugator by a second optical fiber. The second phase conjugator converts the first phase conjugate beam into a second phase conjugate beam. The second phase conjugate beam is transmitted by a third optical fiber. The second optical fiber is composed of a first portion located between the first phase conjugator and a system midpoint and a second portion located between the system midpoint and the second phase conjugator. The total dispersion of the first optical fiber substantially coincides with the total dispersion of the first portion, and the total dispersion of the second portion substantially coincides with the total dispersion of the third optical fiber. By the construction, waveform distortion by chromatic dispersion or nonlinearity is compensated for.

DESCRIPTION

[0001] 1. Technical Field

[0002] This invention relates to an optical fiber communication systemusing optical phase conjugation as well as an apparatus applicable tothe system and a method of producing the same.

[0003] 2. Backaround Art

[0004] As a result of development of a silica optical fiber of low loss,many optical fiber communication systems wherein an optical fiber isused for a transmission line have been put into practical use. Anoptical fiber itself has a very broad band. However, the transmissioncapacity by an optical fiber is actually limited by system designing.The most significant limitation arises from waveform distortion bychromatic dispersion which occurs in an optical fiber. Further, while anoptical fiber attenuates an optical signal, for example, at the rate ofapproximately 0.2 dB/km, the loss by such attenuation has beencompensated for by adoption of optical amplifiers including anerbium-doped fiber amplifier (EDFA).

[0005] Chromatic dispersion often simply called dispersion is aphenomenon wherein the group velocity of an optical signal in an opticalfiber varies as a function of the wavelength (frequency) of the opticalsignal. For example, in a standard single mode fiber, where thewavelength is shorter than 1.3 μm, an optical signal having a longerwavelength propagates faster than another optical signal having ashorter wavelength, and dispersion as a result of this is usually callednormal dispersion. Where the wavelength is longer than 1.3 μm, anoptical signal having a shorter wavelength propagates faster thananother optical signal having a longer wavelength, and dispersion as aresult of this is called anomalous dispersion.

[0006] In recent years, originating from an increase in optical signalpower by adoption of an EDFA, attention is paid to the nonlinearity. Themost significant nonlinearity of an optical fiber which limits thetransmission capacity is an optical Kerr effect. The optical Kerr effectis a phenomenon wherein the refractive index of an optical fiber variesin accordance with the intensity of an optical signal. The variation ofthe refractive index modulates the phase of an optical signal whichpropagates in an optical fiber, and as a result, frequency chirpingwhich varies the signal spectrum occurs. This phenomenon is known asself-phase modulation (SPM). The spectrum is expanded by the SPM, bywhich the waveform distortion by chromatic dispersion is furtherincreased.

[0007] In this manner, the chromatic dispersion and the Kerr effectprovide waveform distortion to an optical signal as the transmissiondistance increases. Accordingly, in order to allow long-haultransmission by an optical fiber, it is required that the chromaticdispersion and the nonlinearity be controlled, compensated for orsuppressed.

[0008] As a technique for controlling the chromatic dispersion and thenonlinearity, a technique which employs a regenerative repeater whichincludes an electronic circuit for a main signal is known. For example,a plurality of regenerative repeaters are disposed intermediately of atransmission line, and in each of the regenerative repeaters,photo-electric conversion, regeneration processing and electro-opticalconversion are performed in this order before the waveform distortion ofthe optical signal becomes excessive. This method, however, has aproblem in that a regenerative repeater which is expensive andcomplicated is required and an electronic circuit of the regenerativerepeater limits the bit rate of a main signal.

[0009] As a technique for compensating for the chromatic dispersion andthe nonlinearity, a light soliton is known. Optical signal pulses havingan amplitude, a pulse width and a peak power defined accurately withrespect to a given value of the anomalous dispersion are generated, andconsequently, a light soliton propagates while it maintains its waveformbecause of balancing between pulse compression which arises from the SPMand the anomalous dispersion by the optical Kerr effect and pulseexpansion by the dispersion.

[0010] As another technique for compensating for the chromaticdispersion and the nonlinearity, application of optical phaseconjugation is available. For example, a method for compensating for thechromatic dispersion of a transmission line has been proposed by Yarivet al. (A. Yariv, D. Fekete, and D. M. Pepper, Compensation for channeldispersion by nonlinear optical phase conjugation” Opt. Lett., vol. 4,pp. 52-54, 1979). An optical signal is converted into phase conjugatelight at a middle point of a transmission line, and waveform distortionby chromatic dispersion which the optical signal has undergone in theformer half of the transmission line is compensated for by distortion bychromatic dispersion in the latter half of the transmission line.

[0011] Particularly, if it is assumed that the factors of the phasevariation of an electric field at two locations are same and thevariation in environment which brings about the factors is moderatewithin a transmission time of light between the two locations, then thephase variation is compensated for by disposing a phase conjugator(phase conjugate light generating apparatus) intermediately between thetwo locations (S. Watanabe, “Compensation of phase fluctuation in atransmission line by optical conjugation” Opt. Lett., vol. 17, pp.1,355-1,357, 1992). Accordingly, by adoption of a phase conjugator, alsowaveform distortion which arises from SPM is compensated for. However,where the distribution of the optical power is asymmetrical before andafter the phase conjugator, the compensation for the nonlinearitybecomes incomplete.

[0012] The inventor of the present invention has proposed a techniquefor overcoming the incompleteness of the compensation by thenonlinearity of the optical power where a phase conjugator is used (S.Watanabe and M. Shirasaki, “Exact compensation for both chromaticdispersion and Kerr effect in a transmission fiber using optical phaseconjugation” J. Lightwave Technol., vol. 14, pp. 243-248, 1996). A phaseconjugator is disposed in the proximity of a point of a transmissionline before and after which the total amounts of the dispersion valuesor the nonlinear effect are equal, and various parameters before andafter the point are set for each small interval. However, since a phaseconjugator is disposed intermediately of the transmission line, wherethe transmission line is laid between continents, for example, the phaseconjugator may possibly be laid on the bottom of the sea. In thisinstance, maintenance of the phase conjugator is difficult. It may beproposed to dispose a front half portion or a rear half portion of atransmission line in a transmission terminal station or a receptionterminal station and lay the remaining half of the transmission linebetween continents. In this instance, since the phase conjugator can beprovided in the transmission terminal station or the reception terminalstation, maintenance of it is easy. However, in this instance, adeviation may appear in setting of parameters between the front halfportion and the rear half portion of the transmission line and may makethe compensation incomplete.

[0013] It is an object of the present invention to provide an opticalfiber communication system wherein the chromatic dispersion and thenonlinearity can be compensated for effectively by using two or morephase conjugators.

[0014] It is another object of the present invention to provide anoptical fiber communication system wherein a phase conjugator need notbe disposed intermediately of a transmission line in order to compensatefor the chromatic dispersion and the nonlinearity.

[0015] Other objects of the present invention become apparent from thefollowing description.

DISCLOSURE OF THE INVENTION

[0016] According to the present invention, there is provided an opticalfiber communication system which includes first and second phaseconjugators. A signal beam is supplied to the first phase conjugator bya first optical fiber. The first phase conjugator converts the signalbeam into a first phase conjugate beam and outputs the first phaseconjugate beam. The first phase conjugate beam is supplied to the secondphase conjugator by a second optical fiber. The second phase conjugatorconverts the first phase conjugate beam into a second phase conjugatebeam and outputs the second phase conjugate beam. The second phaseconjugate beam is transmitted by a third optical fiber. A systemmidpoint is set intermediately of the second optical fiber. Inparticular, the second optical fiber is composed of a first portionlocated between the first phase conjugator and the system midpoint and asecond portion located between the system midpoint and the second phaseconjugator. The total dispersion (product of an average value of thechromatic dispersion and the length) of the first optical fibersubstantially coincides with the total dispersion of the first portion,and the total dispersion of the second portion substantially coincideswith the total dispersion of the third optical fiber. Detailed designexamples of individual parameters are hereinafter described.

[0017] By such parameter setting, the chromatic dispersion and thenonlinearity are compensated for effectively. Further, since thewaveform distortion exhibits a minimum value at the system midpointusing the two phase conjugators, the phase conjugators need not bedisposed intermediately of the transmission line. According to thepresent invention, not only the optical Kerr effect but also othernonlinearities such as a Raman effect are compensated for.

[0018] Preferably, a plurality of optical amplifiers are provided on theoptical path including the first, second and third optical fibers. Evenif noise which is generated by the optical amplifiers is accumulated,according to the present invention, since the waveform of the opticalsignal restores its original waveform once at the system midpoint, thenoise can be removed effectively by an optical band-pass filter in theproximity of the system midpoint. In other words, in the presentinvention, since the signal spectrum at the system midpoint is as narrowas the original signal spectrum, use of an optical band-pass filterhaving a narrow pass-band for removing noise is allowed.

BREIF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a block diagram showing a basic construction of anoptical fiber communication system of the present invention;

[0020]FIG. 2 is a diagrammatic view illustrating a principle ofcompensation in the system of FIG. 1;

[0021]FIG. 3 is a block diagram of a system which was used in ademonstration experiment;

[0022]FIG. 4 is a diagram illustrating a BER (bit error rate)characteristic;

[0023]FIGS. 5A to 5E are diagrams illustrating a variation in waveformof the system of FIG. 3;

[0024]FIG. 6 is a block diagram showing a construction example of aphase conjugator which can be applied to the present invention;

[0025]FIG. 7 is a block diagram showing a first embodiment of an opticalcommunication system of the present invention;

[0026]FIG. 8 is a diagram of an optical power and so forth of the systemof FIG. 7;

[0027]FIG. 9 is a block diagram of an optical amplifier which can beapplied to the present invention;

[0028]FIG. 10 is a block diagram showing a second embodiment of anoptical communication system of the present invention;

[0029]FIG. 11 is a block diagram showing a third embodiment of anoptical communication system of the present invention;

[0030]FIG. 12 is a block diagram showing a fourth embodiment of anoptical communication system of the present invention;

[0031]FIG. 13 is a diagrammatic view showing a dispersion compensatorwhich uses a fiber grating;

[0032]FIG. 14 is a block diagram showing a fifth embodiment of anoptical communication system of the present invention;

[0033]FIG. 15 is a block diagram showing a sixth embodiment of anoptical communication system of the present invention;

[0034]FIG. 16 is a block diagram showing a seventh embodiment of anoptical communication system of the present invention;

[0035]FIGS. 17A and 17B are diagrammatic views showing design examplesof a dispersion parameter in the system of FIG. 16;

[0036]FIGS. 18A and 18B are block diagrams showing examples of anoptical network to which the present invention can be applied;

[0037]FIG. 19 is a block diagram showing another basic construction ofan optical fiber communication system of the present invention;

[0038]FIG. 20 is a diagrammatic view illustrating an embodiment of amanufacturing process of a nonlinear optical medium according to thepresent invention;

[0039]FIG. 21 is a block diagram showing a construction example ofanother phase conjugate light generator which can be applied to thepresent invention;

[0040]FIG. 22 is a diagram illustrating collective conversion of WDM(wavelength division multiplex) signal light by a phase conjugatorhaving a broad conversion band;

[0041]FIG. 23 is a diagrammatic view showing an embodiment of a systemto which wavelength conversion and phase conjugate conversion areapplied;

[0042]FIG. 24 is a diagrammatic view illustrating a setting example of awavelength band in FIG. 23;

[0043]FIG. 25 is a diagrammatic view illustrating another settingexample of a wavelength band in FIG. 23;

[0044]FIG. 26 is a diagrammatic view showing an example of a dispersionarrangement in FIG. 23;

[0045]FIG. 27 is a block diagram showing an improvement to the phaseconjugator shown in FIG. 6;

[0046]FIG. 28A is a diagram illustrating a characteristic of opticalfilters 152, 154 and 156 shown in FIG. 27; and

[0047]FIGS. 28B to 28D are diagrams illustrating spectra observed atdifferent positions of the phase conjugator shown in FIG. 27.

BEST MODE FOR CARRYING OUT THE INVENTION

[0048] In the following, preferred embodiments of the present inventionare described in detail with reference to the accompanying drawings.

[0049] Referring to FIG. 1, there is shown a basic construction of theoptical fiber communication system of the present invention. An opticaltransmitter (OS) 2 outputs a signal beam. A first optical fiber 4 has afirst end 4A and a second end 4B which serve as an input end and anoutput end for the signal beam, respectively. A first phase conjugator(1st PC) 6 is operatively connected to the second end 4B.

[0050] In the present application, the terminology that a certainelement and another element are operatively connected to each otherincludes a case wherein the elements are connected directly to eachother and also includes a case wherein the elements are provided in sucha degree of connection that communication of an optical signal (orelectric signal) is possible between the elements.

[0051] The first phase conjugator 6 converts a signal beam suppliedthereto from the first optical fiber 4 into a first phase conjugate beamand outputs the first phase conjugate beam. A second optical fiber 8 hasa third end 8A and a fourth end 8B which serve as an input end and anoutput end for the first phase conjugate beam, respectively. A secondphase conjugator (2nd PC) 10 is operatively connected to the fourth end8B. The second phase conjugator 10 converts the first phase conjugatebeam supplied thereto from the second optical fiber 8 into a secondphase conjugate beam and outputs the second phase conjugate beam. Athird optical fiber 12 has a fifth end 12A and a sixth end 12B whichserve as an input end and an output end for the second phase conjugatebeam, respectively. In order to receive the second phase conjugate beamtransmitted by the third optical fiber 12, an optical receiver (OR) 14is provided.

[0052] A system midpoint 16 is set intermediately of the second opticalfiber 8. The system midpoint 16 is defined, for example, as a point atwhich waveform distortion is minimized, and a detailed position of thepoint is hereinafter described. The second optical fiber 8 is composedof a first portion 81 located between the third end 8A and the systemmidpoint 16 and a second portion 82 located between the system midpoint16 and the fourth end 8B.

[0053] Parameters of the optical fibers 4, 8 and 12 are set, forexample, in the following manner.

[0054] First, the first optical fiber 4 is imaginarily divided into N (Nis an integer larger than 1) intervals (or sections) 4 (#1, . . . , #N),and also the first portion 81 of the second optical fiber 8 isimaginarily divided into an equal number of intervals 81 (#1, . . . ,#N). In this instance, the first optical fiber 4 and the first portion81 of the second optical fiber 8 are imaginarily divided such thatproducts of average values of chromatic dispersion and interval lengthsof each two mutually corresponding intervals as counted from the firstphase conjugator 6 are equal to each other. More particularly, where theaverage value of chromatic dispersion (or dispersion parameter) and theinterval length of the i-th (1≦i≦N) interval 4 (#i) of the first opticalfiber 4 as counted from the first phase conjugator 6 are represented byD_(1i) and L_(1i), respectively, and the average value of chromaticdispersion (or dispersion parameter) and the interval length of the i-thinterval 81 (#i) of the first portion 81 of the second optical fiber 8as counted from the first phase conjugator 6 are represented by D_(2i)and L_(2i), respectively,

D _(1i) L _(1i) =D _(2i) L _(2i)  (1)

[0055] is satisfied. Further, where the average value of optical powerand the average value of non-linear coefficient of the interval 4 (#i)are represented by P_(1i) and γ_(1i), respectively, and the averagevalue of optical power and the average value of non-linear coefficientof the interval 81 (#i) are represented by P_(2i) and γ_(2i),respectively,

P _(1i)γ_(1i) L _(1i) =P _(2i)γ_(2i) L _(2i)  (2)

[0056] is satisfied.

[0057] Meanwhile, the second portion 82 of the second optical fiber 8 isimaginarily divided into M (M is an integer larger than 1) intervals 82(#1, . . . , #M), and also the third optical fiber 12 is divided into anequal number of intervals 12 (#1, . . . , #M). In this instance, wherethe average value of chromatic dispersion and the interval length of thej-th (1≦j≦M) interval 82 (#i) of the second portion 82 of the secondoptical fiber 8 as counted from the second phase conjugator 10 arerepresented by D_(3j) and L_(3j), respectively, and the average value ofchromatic dispersion and the interval length of the j-th interval 12(#j) of the third optical fiber 12 as counted from the second phaseconjugator 10 are represented by D_(4j) and L_(4j), respectively,

[0058] ti D _(3j) L _(3j) =D _(4j) L _(4j)  (3)

[0059] is satisfied. Further, where the average value of optical powerand the average value of non-linear coefficient of the interval 82 (#j)are represented by P_(3j) and γ_(3j), respectively, and the averagevalue of optical power and the average value of non-linear coefficientof the interval 12 (#j) are represented by P_(4j) and γ_(4j),respectively,

P _(3j)γ_(3j) L _(3j) =P _(4j)γ_(4j) L _(4j)  (4)

[0060] is satisfied.

[0061] In the system of FIG. 1, while the wavelength distortion exhibitsa higher value once before and after the first phase conjugator 6, bythe conditions of the expressions (1) and (2), the chromatic dispersionand the nonlinearity are compensated for at the system midpoint 16, andthe waveform restores its original state once. While the thus restoredwaveform is distorted before and after the second phase conjugator 10again, by the conditions of the Expressions (3) and (4), the chromaticdispersion and the nonlinearity are compensated for at the opticalreceiver 14, and consequently, the waveform restores its originalwaveform again.

[0062] Further, the system of the present invention is tolerant ofsetting errors of parameters such as the length as to the second opticalfiber 8 which may possibly be laid on the bottom of the sea or the like.In particular, even if the waveform does not completely restore itsoriginal state at the system midpoint 16, the waveform can be returnedsubstantially completely to its original waveform at the opticalreceiver 14 by reproducing the incompleteness by the second portion 82,second phase conjugator 10 and third optical fiber 12.

[0063] Referring to FIG. 2, there is illustrated a principle ofcompensation for chromatic dispersion and nonlinearity. Here, aprinciple of compensation from the optical transmitter 2 to the systemmidpoint 16 is described. First, prior to description of FIG. 2, generalmatters of a phase conjugate wave are described.

[0064] Propagation of an optical signal E(x, y, z, t)=F(x, y)φ(z,t)exp[i(ωt−kz)] in optical fiber transmission can generally be describedby a nonlinear wave equation given below. Here, F(x, y) represents themode distribution in a lateral direction and φ(z, t) represents thecomplex envelope of light. It is assumed that φ(z, t) here variessufficiently slowly comparing with the frequency ω of the light.$\begin{matrix}{{{i\quad \frac{\partial\varphi}{\partial z}} - {\left( {1/2} \right)\beta_{2}\quad \frac{\partial^{2}\varphi}{\partial T^{2}}}\quad + {\gamma {\varphi }^{2}\varphi}} = {{- \left( {i/2} \right)}{\alpha\varphi}}} & (5)\end{matrix}$

[0065] where T=t−β₁z (β₁ is a propagation constant), α is the loss ofthe fiber, β₂ is the chromatic dispersion of the fiber, and$\begin{matrix}{\gamma = \frac{\omega \quad n_{2}}{{cA}_{eff}}} & (6)\end{matrix}$

[0066] represents the third-order nonlinear coefficient (coefficient ofan optical Kerr effect). Here, n₂ and A_(eff) represent the nonlinearrefractive index and the effective core sectional area of the fiber,respectively. c represents the velocity of light in the vacuum. Here, upto first-order dispersion is taken into consideration, and higher orderdispersion is omitted. Further, it is assumed that α, β₂ and γ arefunctions of z, which are represented as α(z), β₂(z) and γ(z),respectively. Furthermore, the position of the phase conjugator isdetermined as an origin (z=0). Here, the following normalizationfunction is used:

φ(z, T)=A(z)u(z, T)  (7)

[0067] where

A(z)≡A(0)exp[−(1/2)∫₀ ^(z)α(z)dz]  (8)

[0068] represents the amplitude, and where α(z)>0, this represents thatthe transmission line has a loss, but where α(z)<0, this represents thatthe transmission line has a gain. A(z)≡A(0) represents the case wherethe transmission has no loss. Meanwhile, A(z)²=P(z) corresponds to theoptical power. By substituting the expressions (7) and (8) into theexpression (5), the following development equation is obtained.$\begin{matrix}{{i\quad \frac{\partial u}{\partial z}} = {{\left( {1/2} \right){\beta_{2}(z)}\quad \frac{\partial^{2}u}{\partial T^{2}}} - {{\gamma (z)}{A(z)}{u}^{2}u}}} & (9)\end{matrix}$

[0069] Here, the following transformation is performed:

ζ=∫₀ ^(z)|β₂(z)|dz  (10)

[0070] As a result, the expression (9) can be transformed into thefollowing manner: $\begin{matrix}{{i\quad \frac{\partial u}{\partial\zeta}} = {{\frac{{sgn}\left\lbrack \beta_{2} \right\rbrack}{2}\quad \frac{\partial^{2}u}{\partial T^{2}}} - {\frac{{\gamma (\zeta)}{A(\zeta)}^{2}}{{\beta_{2}(\zeta)}}{u}^{2}u}}} & (11)\end{matrix}$

[0071] where sgn[β₂]≡±1 assumes +1 when β₂>0, that is, when thedispersion is normal dispersion, but assumes −1 when β₂<0, that is, whenthe dispersion is anomalous dispersion. If the expression (11) stands,then also a complex conjugate with it stands, and the followingexpression is obtained. $\begin{matrix}{{{- i}\quad \frac{\partial u^{*}}{\partial\zeta}} = {{\frac{{sgn}\left\lbrack \beta_{2} \right\rbrack}{2}\quad \frac{\partial^{2}u^{*}}{\partial T^{2}}} - {\frac{{\gamma (\zeta)}{A(\zeta)}^{2}}{{\beta_{2}(\zeta)}}{u^{*}}u^{*}}}} & (12)\end{matrix}$

[0072] Complex conjugate light u^(*) behaves in accordance with adevelopment equation same as the development equation for u. However,the propagation direction then is opposite. This operation precisely isoperation of a phase conjugator. Particularly in a phase conjugator ofthe transmission type, the above operation is equivalent to inversion ofa phase shift by chromatic dispersion and SPM.

[0073] Here, in FIG. 2, it is assumed that the length of the firstoptical fiber 4 is L₁ and the length of the first portion 81 of thesecond optical fiber 8 is L₂. Further, the first phase conjugator 6 isdisposed at the origin z=0 (ζ=0) of the z coordinate and the ζcoordinate. The z coordinate and the ζ coordinate of the system midpoint16 are L₂ and ζ₀, respectively.

[0074] In the first optical fiber 4, a signal beam u (Es) propagates inaccordance with the development equation (11). The signal beam u isconverted into a phase conjugate beam u^(*) (Ec) by the first phaseconjugator 6. The phase conjugate beam u^(*) propagates in accordancewith the development equation (12) in the first portion 81 of the secondoptical fiber 8. In this instance, if the values of the parameters areset so that the coefficients of the first and second terms of the rightside of the expression (11) are equal within a normalized distance dζfor two arbitrary points −ζ and ζ located at symmetrical positions onthe ζ axis with respect to the position (ζ=0) of the first phaseconjugator 6, then u^(*) at −ζ becomes a phase conjugate wave of u at ζ.In particular, the following two expressions become requirements.

sgn[β₂(−ζ)]=sgn[β₂(ζ)]  (13)

[0075] $\begin{matrix}{\frac{{\gamma \left( {- \zeta} \right)}{A\left( {- \zeta} \right)}^{2}}{{\beta_{2}\left( {- \zeta} \right)}} = \frac{{\gamma (\zeta)}{A(\zeta)}^{2}}{{\beta_{2}(\zeta)}}} & (14)\end{matrix}$

[0076] The expression (13) indicates the necessity that the signs of thedispersions of the first optical fiber 4 and the first portion 81 beequal to each other. If it is taken into consideration that γ>0 andA(z)²>0 in a fiber, then the requirements given above can be gathered inthe following manner. $\begin{matrix}{\frac{{\gamma \left( {- \zeta} \right)}{A\left( {- \zeta} \right)}^{2}}{\beta_{2}\left( {- \zeta} \right)} = \frac{{\gamma (\zeta)}{A(\zeta)}^{2}}{\beta_{2}(\zeta)}} & (15)\end{matrix}$

[0077] The phase shift by the chromatic dispersion and the SPM at (−ζ)in the first optical fiber 4 is inverted in sign by the first phaseconjugator 6. Accordingly, waveform distortion by the phase shift iscompensated for by distortion by the phase shift at (ζ) in the firstportion 81. If compensation by such setting as described above isrepeated for each interval in this manner, then compensation over theoverall length is possible.

[0078] Next, the compensation requirement described above is describedin connection with the z coordinate. From the expression (15),$\begin{matrix}{\frac{{\gamma \left( {- z_{1}} \right)}{A\left( {- z_{1}} \right)}^{2}}{\beta_{2}\left( {- z_{1}} \right)} = \frac{{\gamma \left( z_{2} \right)}{A\left( z_{2} \right)}^{2}}{\beta_{2}\left( z_{2} \right)}} & (16)\end{matrix}$

[0079] is obtained. In particular, to make the ratios of the chromaticdispersions to products of the nonlinear coefficients and the opticalpowers in the individual intervals equal to each other becomes arequirement. Here, −z₁ and z₂ represent two points which satisfy thefollowing expression.

∫₀ ^(−z1)|β₂(z)|dz=−∫ ₀ ^(z2)|β₂(z)|ds  (17)

[0080] From the expressions (16) and (17), expressions (18) and (19) areobtained:

β₂(−z ₁)dz ₁=β₂(z ₂)dz ₂  (18)

γ(−z ₁)A(−z ₁)² dz ₁=γ(z ₂)A(z ₂)² dz ₂  (19)

[0081] where dz₁ and dz₂ are lengths of small internvals at −z1 and z2,respectively, and each interval length increases in inverse proportionto the dispersion in the interval or in inverse proportion to theproduct of the nonlinear coefficient and the optical power. Here, if therelationship between the dispersion β₂ and the dispersion parameter D,that is, D=−(2πc/λ²)β₂, is taken into consideration, then a relationshipgiven below is obtained from the expressions (18) and (19). D is afunction of z and is represented also as D(z).

D(−z ₁)dz ₁ =D(z ₂)dz ₂  (20)

γ(−z ₁)P(−z ₁)dz ₁=γ(z ₂)P(z ₂)dz ₂  (21)

[0082] It can be seen that it is a requirement for compensation for bothof the dispersion and the nonlinearity that an increment at one of twopositions symmetrical with respect to the first phase conjugator 6 beequal to a decrement at the other of the two positions.

[0083] The expressions (20) and (21) are requirements for compensationand indicate that the total dispersion amounts and the total amounts ofthe Kerr effect in two intervals corresponding to each other are equalto each other. Thus, the effectiveness of the conditions of theexpressions (1) to (4) are confirmed.

[0084] Particularly where α, D and γ are substantially fixed and thevariation of the power is small, by integrating the expressions (20) and(21),

D ₁ L ₁ =D ₂ L ₂  (22)

γ₁ {overscore (P)} ₁ L ₁=γ₂ {overscore (P)} ₂ L ₂  (23)

[0085] are obtained. Here, {overscore (P)}₁ and {overscore (P)}₂ areaverage powers in the first optical fiber 4 and the first portion 81,respectively. Further, D₁ and γ₁ are the dispersion parameter and thenonlinear coefficient of the first optical fiber 4 or average values ofthem, respectively, and D₂ and γ₂ are the dispersion parameter and thenonlinear coefficient of the first portion 81 or average values of them,respectively. The expressions (22) and (23) coincide with requirementsin an SPM compensation method by dispersion compensation and averagevalue approximation.

[0086] In practical use, the present invention can be worked only if therequirement of the expression (22) is satisfied. For example, the systemof FIG. 1 may be constructed such that the product of the average valueof the chromatic dispersion and the length of the first optical fiber 4is substantially equal to the product of the average value of thechromatic dispersion and the length of the first portion 81 of thesecond optical fiber 8 and the product of the average value of thechromatic dispersion and the length of the second portion 82 of thesecond optical fiber 8 is substantially equal to the product of theaverage value of the chromatic dispersion and the length of the thirdoptical fiber 12. By this setting, the wavelength distortion by thechromatic dispersion is compensated for.

[0087] Preferably, in order to further satisfy the requirement of theexpression (23), the product of the average value of the optical power,the average value of the nonlinear coefficient, and the length of thefirst optical fiber 4 is made substantially equal to the product of theaverage value of the optical power, the average value of the nonlinearcoefficient, and the length of the second portion 81, and the product ofthe average value of the optical power, the average value of thenonlinear coefficient, and the length of the second portion 82 is madesubstantially equal to the product of the average value of the opticalpower, the average value of the nonlinear coefficient, and the length ofthe third optical fiber 12. By this setting, in addition to the waveformdistortion by the chromatic dispersion, also the waveform distortion bythe nonlinearity is compensated for.

[0088] Where a plurality of optical amplifiers are provided on theoptical path which includes the first, second and third optical fibers4, 8 and 12, preferably the distance between each adjacent ones of theoptical amplifiers is set shorter than the nonlinear length of theoptical path (optical fiber). The nonlinear length is hereinafterdescribed.

[0089] In FIG. 2, a principle of compensation on the upstream side ofthe system midpoint 16 is illustrated. Since the principle ofcompensation on the downstream side of the system midpoint 16 can berecognized similarly, description of it is omitted herein.

[0090] In the description with reference to FIG. 2, as seen from theexpression (10), a normalized coordinate is defined by an accumulatedvalue of chromatic dispersion from the phase conjugator 6. As a result,it is a required condition that the ratios between the products of theoptical powers and the nonlinear coefficients and the chromaticdispersions at two points on the first optical fiber 4 and the firstportion 81 at which the accumulated values of the chromatic dispersionsfrom the first phase conjugator 6 are equal to each other.

[0091] In FIG. 2, a normalized coordinate may be defined by anaccumulated value of the nonlinear effect from the first phaseconjugator 6 (that is, an accumulated value of the products of theoptical powers and the nonlinear coefficients). In this instance, it isa requirement that the ratios between the chromatic dispersions and theproducts of the optical powers and the nonlinear coefficients at twopoints on the first optical fiber 4 and the first portion 81 at whichthe accumulated values from the first phase conjugator 6 are equal toeach other be substantially equal to each other.

[0092] In the following, a result of an experiment conducted todemonstrate the effectiveness of the principle of FIG. 2 is described.

[0093] Referring to FIG. 3, there is shown a block diagram of a systemused in the demonstrating experiment.

[0094] A transmitter (Transmitter) corresponds to the opticaltransmitter 2 of FIG. 1; a fiber compensator (Fiber compensator)corresponds to the first optical fiber 4 of FIG. 1; a phase conjugator(Phase conjugator) corresponds to the first phase conjugator 6 of FIG.1; and dispersion shifted fibers (DSF-1, 2, . . . , 46) and erbium-dopedfiber amplifiers (EDFA1, 2, . . . , 45) correspond to the first portion81 of the second optical fiber 8 of FIG. 1. A receiver (Receiver) formeasuring a transmission characteristic is provided at the systemmidpoint 16 of FIG. 1.

[0095] For a light source of the transmitter, two DFB-LDs (distributedfeedback type laser diodes) of the 3-electrodes γ/4 shift type wereused. Time division multiplexed signal light Es (wavelength γs=1,551 nm)of 20 Gb/s was produced by time division multiplexing RZ signals of 2channels of 10 Gb/s having a pulse width (FWHM) of approximately 40 ps.In order to produce RZ pulses of 10-Gb/s, Es was intensity modulatedwith a sine wave of 10 GHz using a first LiNbO₃ modulator (LN-1) andthen intensity modulated with an NRZ data signal (PN: 2²³−1) of 10 Gb/susing a second LiNbO₃ modulator (LN-2). The modulated Es was inputted toDD-DCF1 and DD-DCF2 of two stages so that the waveform was compensatedfor in advance.

[0096] Here, the “DD-DCF” represents a dispersion compensating fiber ofthe dispersion gradually decreasing type (dispersion-decreasingdispersion-compensating fiber: DD-DCF).

[0097] Each of the DD-DCFs is formed from five DCFs (DCF-a, b, c, d ande) spliced to each other. The loss of each of the DD-DCFs was 0.46dB/km, and the mode field diameter of each of the DCFs was set toapproximately 4 μm.

[0098] In order to approximately satisfy the requirement of theexpression (16), the dispersion parameter D₁ should decrease inaccordance with a decrease of the average optical power in each of theDD-DCFs. To this end, the length and D₁ of each of the five DCFs was setin such a manner as seen in Table below: TABLE DCF Length (km) D₁(ps/nm/km) a 2.8 −80.6 b 2.7 −57.9 c 2.8 −43.7 d 2.7 −32.1 e 2.7 −27.0

[0099] The length of each of the DD-DCFs was 13.7 km, and the totaldispersion of each of the DD-DCFs was −662.8 ps/nm.

[0100] It is to be noted that, in order to set the power of light to beinputted to each of the DD-DCFs to P₁, two optical amplifiers wereconnected in cascade connection.

[0101] Then, the phase conjugator converted Es compensated for inadvance (provided with distortion) into phase conjugate light Ec(wavelength λc=1,557 nm), which propagated in the same direction as Es,by forward FWM (four wave mixing) of the non-degeneration type usingpump light Ep of a wavelength λp=1,554 nm in a DSF of 20 km. Theconversion efficiency from Es to Ec was −12 dB.

[0102] Then, the phase conjugate light Ec was supplied to a transmissionline of 3,036 km formed from 46 DSFs (0.21 dB/km in loss) connected incascade connection and 45 EDFAs (each having a noise figure ofapproximately 6 dB) interposed between the DSFs. The average dispersionat λc of this transmission line was −0.44 ps/nm/km. Accordingly, thedifference between the total dispersion of the DD-DCFs of the two stagesand the total dispersion of the transmission line was approximately 10ps/m. The length of each of the DSFs was 66 km, and the optical inputpower P₂ to each DSF was set to +6 dBm.

[0103] The optimum value of P₁ was, in the conditions described above,+16 dBm. The nonlinear coefficient γ₁ of the DD-DCFs was estimated to beapproximately 18.0 W⁻¹km⁻¹.

[0104] In order to suppress stimulated brillouin scattering (SBS), Esand Ep were frequency modulated with sine wave signals of 500 kHz and150 kHz, respectively. In the receiver, a third LiNbO₃ modulator (LN-3)and a phase-locked loop (PLL) were used to time division demultiplex Ecand measure the bit error rate (BER).

[0105] For comparison, also a transmission experiment over 1,518 kmusing one DD-DCF and 23 DSFs was conducted.

[0106] A characteristic of the BER measured is illustrated in FIG. 4.Even after the transmission of 3,036 km, the signal was detectedsuccessfully with a BER lower than 10⁻⁹. The power penalty of 4.8 dB inthe BER of 10⁻⁹ originated from S/N deterioration from a theoreticalvalue by noise of the EDFA and so forth. In the experiment, λc detunedby approximately 1.5 nm from the wavelength λG≈1,558.5 nm with which again peak is exhibited for each EDFA. If it is possible to make λccoincide with λG, then a higher S/N characteristic can be obtained. Inthe transmission experiment of 1,518 km, the penalty was approximately1.2 dB.

[0107]FIGS. 5A to 5E illustrate a manner of variation of the waveformdetected in a 3,036 km transmission experiment. FIG. 5A shows an outputwaveform of the transmitter; FIG. 5B shows an output waveform of thephase conjugator; FIG. 5C shows a waveform after transmission over 1,518km; FIG. 5D shows a waveform after transmission over 2,706 km; and FIG.5E shows a waveform after transmission over 3,036 km. It can be seenthat a waveform distorted in advance is gradually improved as Ecpropagates. The residue of waveform distortion in FIG. 5E originatedfrom an incomplete compensation condition. In particular, in thedemonstration experiment, due to the fact that the distance between theEDFAs (the length of each DSF: 66 km) was not sufficiently shorter thanthe nonlinear length defined by a reciprocal number to the product ofthe nonlinear coefficient and the optical power, the improvement inwaveform was not complete.

[0108] Accordingly, in the present invention, where a plurality ofoptical amplifiers are used, it is preferable to set the distancebetween them shorter than the nonlinear length.

[0109] Further, the compensation can be further improved by setting thedividing number of DCFs in a DD-DCF larger than 5 used in theexperiment.

[0110] For each of the optical fibers 4, 8 and 12 of FIG. 1, a singlemode silica fiber can be used. Silica fibers for use with optical fibercommunication may be 1.3 μm zero dispersion fibers, 1.55 μm dispersionshifted fibers and so forth.

[0111] For a modulation method for signal light by the opticaltransmitter 2, optical amplitude (intensity) modulation, frequencymodulation, phase modulation or any other available modulation methodcan be used. Further, for signal detection by the optical receiver 14,optical direct detection after filtering by an optical band filter oroptical heterodyne detection can be used.

[0112] Each of the phase conjugators 6 and 10 has a second- orthird-order nonlinear optical medium and means for pumping the medium.Where a second-order nonlinear optical medium is used, phase conjugateconversion is performed by a parametric effect, but where a third-ordernonlinear optical medium is used, phase conjugate conversion isperformed by four wave mixing of the degeneration type or thenon-degeneration type.

[0113] For a third-order nonlinear optical medium, for example, a silicafiber can be used. In this instance, good phase conjugate conversion canbe achieved by making the wavelength of pump light for four wave mixingsubstantially coincident with the zero dispersion wavelength of thesilica fiber. A phase conjugator which uses a silica fiber is superiorin high speed, broad band, low distortion and consistency with atransmission line.

[0114] For a third-order nonlinear optical medium, also a semiconductoroptical amplifier (SOA) may be used. A phase conjugator which employs anSOA is superior in broad band and miniaturization.

[0115] For a third-order nonlinear optical medium, a laser diode of thedistribution feedback type (DFB-LD) itself may be used. By injection ofcurrent, the DFB-LD produces pump light, and phase conjugate conversionis performed by four wave mixing. Accordingly, an external pump lightsource is not required. A phase conjugator which employs a DFB-LD issuperior in broad band and miniaturization. For details of a phaseconjugator which employs a DFB-LD, a document (H. Kuwatsuka, H. Shoji,M. Matsuda and H. Ishikawa, “THz frequency conversion usingnondegenerate four-wave mixing process in a lasing long-cavityλ/4-shifted DFB laser” Electron. Lett., vol. 31, pp. 2,108-2,110, 1995)should be referred to.

[0116] For a second-order nonlinear optical medium, an optical waveguidemade of LiNbO₃, AlGaAs or the like can be used. A phase conjugator whichemploys this optical waveguide allows good phase consistency by adoptionof a pseudo phase matching structure and is superior in broad band, andextraction of a phase conjugate-beam therefrom is easy. For this, forexample, a document (C. Q. Xu, H. Okayama and M. Kawahara, “1.5 μm bandefficient broadband wavelength conversion by difference frequencygeneration in a periodically domain-inverted LiNbO3 channel waveguide”Appl. Phys. Lett., vol. 63, No. 26, pp. 3,559-3,561, 1993) should bereferred to.

[0117] Referring to FIG. 6, there is shown a phase conjugator which canbe used for each of the phase conjugators 6 and 10 of FIG. 1. This phaseconjugator includes an optical fiber 18 serving as a third-ordernonlinear optical medium, a laser diode (LD) 20 serving as a pump lightsource, and an optical coupler 22 for adding an input beam and pumplight to each other and supplying the resulting beam to the opticalfiber 18.

[0118] Preferably, the optical fiber 18 is a single mode fiber. In thisinstance, where it is intended to make the wavelength of the input beamand the wavelength of the pump light a little different from each otherso as to cause four wave mixing of the nondegeneration type to occur,the zero-dispersion wavelength of the optical fiber 18 is set equal tothe wavelength of the pump light (oscillation frequency of the LD 20).The optical coupler 22 has four ports 22A, 22B, 22C and 22D. An inputbeam (signal beam or first phase conjugate beam) is supplied to the port22A, and the port 22B is connected to the LD 20. Further, the port 22Cis connected to a first end of the optical fiber 18, and the port 22D ismade a dead end. A second end of the optical fiber 18 serves as anoutput port of the phase conjugator. The optical coupler 22 outputs aninput beam and pump light supplied to the ports 22A and 22B thereof,respectively, from the port 22C thereof. For the optical coupler 22, forexample, an optical coupler of the fiber fusion type, a half mirror, anoptical wave combiner, a polarizing beam splitter or the like is used.

[0119] Referring to FIG. 7, there is shown a first embodiment of thepresent invention. For the first optical fiber 4, two such DD-DCFs 24 aswere used in the demonstration test are adopted. An optical amplifier 26is provided on the input side of each of the DD-DCFs 24 so that thepower of a signal beam to be supplied to each DD-DCF 24 may have apredetermined level. The first portion 81 of the second optical fiber 8is formed from a plurality of optical fibers 28 connected in cascadeconnection. Between each adjacent ones of the optical fibers 28, anoptical amplifier 30 is provided in order to keep the optical power inthe first portion 81 substantially constant. The second portion 82 ofthe second optical fiber 8 is formed from a plurality of optical fibers32. Between each adjacent ones of the optical fibers 32, an opticalamplifier 34 is provided in order to keep the optical power in thesecond portion 82 substantially constant.

[0120] Particularly in the present embodiment, at the system midpoint16, an optical amplifier 36 by which removal of noise is performedeffectively is provided. For the third optical fiber 12, two suchDD-DCFs 38 as are same as those used in the demonstration experiment areadopted. On the input side of each of the DD-DCFs 38, an opticalamplifier 40 is provided in order that the power of a second phaseconjugate beam to be supplied to each DD-DCF 38 may have a leveldetermined in advance.

[0121] The optical transmitter 2, first optical fiber 4 and first phaseconjugator 6 are included in a first terminal station 42, and the secondphase conjugator 10, third optical fiber 12 and optical receiver 14 areincluded in a second terminal station 44. The terminal stations 42 and44 are installed, for example, on different continents from each other,and in this instance, the second optical fiber 8 can be laid as atransmission line on the bottom of the sea between the continents.

[0122] Referring to FIG. 8, there is shown a diagram of optical powersand so forth in the system of FIG. 7. In each of the two DD-DCFs 24which form the first optical fiber 4, the chromatic dispersion β₂gradually decreases as the nonlinear effect (product of the nonlinearcoefficient γ and the optical power P) decreases, and consequently, theratio (γP/β₂) between the nonlinear effect and the chromatic dispersionis substantially fixed.

[0123] Further, intermediately of the second optical fiber 8, aplurality of optical amplifiers 30, 34 and 36 for making the opticalpower in the second optical fiber 8 substantially constant are provided.Accordingly, according to the present embodiment, existing optical fibertransmission lines whose parameters are not designed specifically can beused or combined to form the second optical fiber 8. Details aredescribed below.

[0124] Now, it is assumed that, as the first portion 81 of the secondoptical fiber 8, an existing transmission line composed of a pluralityof optical fibers 28 and a plurality of optical amplifiers 30 as shownin FIG. 7 is provided. Since generally the average value of chromaticdispersion in an existing transmission line is fixed, the ratio (γP/β₂)between the nonlinear effect and the chromatic dispersion in the firstportion 81 of the second optical fiber 8 can be set to a value x givenin advance by suitably setting the gain of each of the opticalamplifiers 30. Once the ratio x is given with regard to the transmissionline, the distribution of the product γP of the nonlinear coefficientand the optical power and the distribution of the chromatic dispersionβ₂ in each of the DD-DCFs 24 are set. Then, the ratio (γ P/β₂) betweenthe nonlinear effect and the chromatic dispersion of the first opticalfiber 4 can thereby be made coincident with the ratio x regarding thefirst portion 81 of the second optical fiber 8. As a result, thewaveform restores its original waveform at the system midpoint 16.

[0125] It is to be noted that, although the system here is designed sothat the fixed ratio x may be obtained with regard to the overall lengthof the first optical fiber 4 and the first portion 81 of the secondoptical fiber 8, where, for example, the optical fibers 28 which formthe first portion 81 have individually different chromatic dispersionsβ₂, since a plurality of intervals having different chromaticdispersions are produced in the first portion 81, the waveform canrestore its original waveform at the system midpoint 16 by imaginarilydividing the first optical fiber 4 into a plurality of intervals andmaking the requirement described hereinabove be satisfied for each twocorresponding intervals in accordance with the present invention.

[0126] The waveform can restore its original waveform also at theoptical receiver 14 by designing the second portion 82 of the secondoptical fiber 8 and the third optical fiber 12 in a similar manner asdescribed above. While, in the example of FIG. 8, the diagram is shownsuch that the first portion 81 and the second portion 82 of the secondoptical fiber 8 have chromatic dispersions of equal values, also wherethey have different chromatic dispersions, the waveform can restore itsoriginal waveform at the optical receiver 14 by suitably setting thegains of the optical amplifiers 40 and the construction of the DD-DCFs38 in the second terminal station 44.

[0127] In this manner, with the present embodiment, by using the secondoptical fiber 8 as a transmission line, construction of a very long-haultransmission system wherein the chromatic dispersion and thenonlinearity are compensated for is allowed. Further, also a singlephase conjugator which is provided intermediately of the transmissionline is not required by providing the phase conjugators 6 and 10 in theterminal stations 42 and 44, respectively, the maintenance feasibilityof the system is improved. In particular, while, taking it intoconsideration that maintenance of a transmission line laid on the bottomof the sea once is very difficult, there is a demand that a phaseconjugator which generally has a complicated construction be notprovided intermediately of a transmission line, the present inventionsatisfies such a demand.

[0128] It is to be noted that, in the system of FIG. 7, in order toachieve improvement of the waveform at the system midpoint 16, it isdesirable to make the distance between the optical amplifiers 30sufficiently shorter than the nonlinear length given as a reciprocalnumber to the product of the nonlinear coefficient and the opticalpower. Similarly, in order to achieve improvement of the waveform at theoptical receiver 14, it is desirable to make the distance between theoptical amplifiers 34 sufficiently shorter than the nonlinear length. Inshort, by making the distance between optical amplifiers sufficientlyshorter than the nonlinear length, the optical power can be handled asbeing constant (average in power) over the overall length. In thisinstance, although the dispersion of the second optical fiber 8 isfixed, the condition that the ratio between the chromatic dispersion andthe nonlinear effect is fixed before and after the phase conjugatorapproximately stands.

[0129] By the way, in the system of FIG. 7, since a plurality of opticalamplifiers are used, noise is accumulated. For example, where eachoptical amplifier is an EDFA, noise by ASE (Amplified SpontaneousEmission) produced in EDFs (erbium-doped fibers) is accumulated.

[0130] In the present invention, as seen in FIG. 2, a signal spectrumgradually expands in the first optical fiber 4, and then the signalspectrum is reversed once on the frequency axis by the first phaseconjugator 6, whereafter the signal spectrum gradually narrows in thefirst portion 81 of the second optical fiber and becomes narrowest atthe system midpoint 16. Accordingly, in the present invention, noise byASE can be removed effectively at the system midpoint 16.

[0131] Referring to FIG. 9, there is shown an optical amplifier whichcan be applied to the system of the present invention. To a first end ofan EDF 46 serving as an optical amplification medium, a beam to beamplified and a first pump beam from a laser diode 50 are suppliedthrough an optical coupler 48. To a second end of the EDF 46, a secondpump beam from a laser diode 54 is supplied through an optical coupler52. When the beam to be amplified is supplied to the EDF 46 which ispumped by the first and second pump beams, the beam is amplified by andoutputted from the amplifier through the optical coupler 52 and anoptical band-pass filter 56. Since ASE generated in the EDF 46 has asufficiently broader band than the amplified beam, most of the ASE canbe removed by the optical band-pass filter 56 to suppress a drop of S/Nof the amplified beam.

[0132] Where, for example, such an optical amplifier as shown in FIG. 9is applied to the optical amplifier 36 provided at the system midpoint16 in the system of FIG. 7, since the signal spectrum is narrowest atthe system midpoint 16, accumulated noise by ASE can be removedefficiently by using an optical band-pass filter which has a pass-band alittle broader than the band-width of the signal spectrum as the filter56.

[0133] It is to be noted that, while, in the optical amplifier of FIG.9, the two laser diodes 50 and 54 are used to pump the EDF 46, only oneof the laser diodes may be used to pump the EDF 46.

[0134] In this manner, with the preferred embodiment of the presentinvention, deterioration in S/N can be prevented effectively byproviding an optical band-pass filter having a pass-band including thewavelength of a first phase conjugate beam in the proximity of thesystem midpoint 16 of the second optical fiber 8.

[0135] Referring to FIG. 10, there is shown an optical communicationsystem showing a second embodiment of the present invention. The presentembodiment is characterized, in contrast with the basic construction ofFIG. 1, in that a branching unit 58 is provided at the system midpoint16 of the second optical fiber 8.

[0136] A signal beam outputted from the optical transmitter 2 issupplied to the first phase conjugator 6 by the first optical fiber 4.The first phase conjugator 6 converts the received signal beam into andoutputs a phase conjugate beam. The phase conjugate beam outputted fromthe first phase conjugator 6 is supplied to the branching unit 58 by thefirst portion 81 of the second optical fiber 8. The branching unit 58branches the received phase conjugate beam into first and second branchbeams. The first and second branch beams are supplied to phaseconjugators 10-1 and 10-2 through second portions 82-1 and 82-2 of thesecond optical fiber 8, respectively. The phase conjugator 10-1 convertsthe received first branch beam into a phase conjugate beam and sends thephase conjugate beam to an optical receiver 14-1 through an opticalfiber (third optical fiber) 12-1. The phase conjugator 10-2 converts thereceived second branch beam into a phase conjugate beam and supplies thephase conjugate beam to an optical receiver 14-2 through an opticalfiber (third optical fiber) 12-2.

[0137] Parameter setting of the optical fibers 4 and 81, parametersetting of the optical fibers 82-1 and 12-1, and parameter setting ofthe optical fibers 82-2 and 12-2 are performed in a similar manner as inthose of FIG. 1 in accordance with the present invention.

[0138] Since the branching unit 58 is provided at the system midpoint16, a transmission characteristic of a phase conjugate beam received canbe monitored by the branching unit 58. To this end, a monitor circuit 60is additionally provided for the branching unit 58. Though not shown, anoptical receiver may be connected to the branching unit 58.

[0139] For example, the optical transmitter 2, first optical fiber 4 andfirst phase conjugator 6 are provided on a first continent; the phaseconjugator 10-1, optical fiber 12-1 and optical receiver 14-1 areprovided on a second continent; the phase conjugator 10-2, optical fiber12-2 and optical receiver 14-2 are provided on a third continent; andthe branching unit 58 and the monitor circuit 60 are provided on anisland between the continents. The branching unit 58 may not be providedprecisely at the system midpoint 16, and under the condition that thewaveform is improved sufficiently, the branching unit 58 may be providedat a location spaced by a certain distance from the system midpoint 16.

[0140] While the second embodiment of the present invention is describedhere in contrast with the basic construction of FIG. 1, the firstembodiment of FIG. 7 may be applied to the second embodiment of FIG. 10.Further, while, in FIG. 10, the branching unit 58 outputs first andsecond branch beams, a phase conjugate beam received by the branchingunit 58 may be branched into three or more branch beams while phaseconjugators and optical receivers corresponding to the branch beams areadditionally provided on the downstream side of the branching unit 58.

[0141] Referring to FIG. 11, there is shown a third embodiment of thepresent invention. Here, in order to expand and apply the basicconstruction of FIG. 1 to WDM (wavelength division multiplex), anoptical multiplexer (MUX) 62 and an optical demultiplexer (DE-MUX) 64are used.

[0142] Optical transmitters 2-1, . . . , n (n is an integer largerthan 1) individually output signal beams having different wavelengthsfrom each other. The signal beams are supplied to the opticalmultiplexer 62 through optical fibers 4-1, . . . , n individuallycorresponding to the first optical fiber 4 of FIG. 1. The opticalmultiplexer 62 wavelength division multiplexes the received signal beamsand outputs a WDM signal beam. Then, the WDM signal beam is supplied tothe first phase conjugator 6. Here, since the optical fibers 4-1, . . ., n for exclusive use are provided individually for the opticaltransmitters 2-1, . . . , n, setting of parameters according to thepresent invention is possible for each wavelength channel. In otherwords, since the nonlinear coefficient and the chromatic dispersion aredifferent among different wavelength channels, according to the presentembodiment, precise compensation for each wavelength channel ispossible.

[0143] The WDM signal beam after phase conjugate converted by the phaseconjugator 6 is supplied through the second optical fiber 8 to thesecond phase conjugator 10, by which it is further phase conjugateconverted. The output beam of the second phase conjugator 10 is suppliedto the optical demultiplexer 64. The optical demultiplexer 64demultiplexes the received beam for the individual wavelength channels,and the beams of the channels are supplied to optical receivers 14-1, .. . , n through optical fibers 12-1, . . . , n, respectively, whichcorrespond to the third optical fiber 12 of FIG. 1. Parameter setting ofeach of the optical fibers 4-1, . . . , n and the first portion 81 ofthe second optical fiber 8 is performed in a similar manner as in thebasic construction of FIG. 1, and also parameter setting of the secondportion 82 of the second optical fiber 8 and each of the optical fibers12-1, . . . , n is performed in a similar manner as in that of FIG. 1.

[0144] While, in the present embodiment, the optical demultiplexer 64 isused in order to demultiplex a beam outputted from the second phaseconjugator 10 into n channels, where a single optical receiver is used,the optical demultiplexer 64 is not necessary. In this instance, theoptical receiver has optical or electric means for selecting a desiredchannel from among the n channels.

[0145] It is to be noted that, while the third embodiment is describedin contrast with the basic construction of FIG. 1, the first embodimentof FIG. 7 may be applied to the third embodiment.

[0146] Referring to FIG. 12, there is shown a fourth embodiment of thepresent invention. Here, in contrast with the basic construction of FIG.1, at least one dispersion compensator (DC) 66 for providing chromaticdispersions of signs opposite to those of the chromatic dispersions ofthe optical fibers 4, 8 and 12 is provided additionally. While, in theexample shown, the dispersion compensator 66 is provided intermediatelyof the optical fiber 8 between the phase conjugators 6 and 10, thedispersion compensator 66 may be connected to the input end or theoutput end of the optical fiber 8. Further, the dispersion compensator66 may be provided intermediately of the optical fiber 4 or connected tothe input end or the output end of the optical fiber 4 or may beprovided intermediately of the optical fiber 12 or connected to theinput end or the output end of the optical fiber 12.

[0147] For the dispersion compensator 66, a dispersion compensationfiber (DCF) having a chromatic dispersion of a high absolute value canbe used. Whether the dispersion of each of the optical fibers 4, 8 and12 is a normal dispersion or an anomalous dispersion, since the lengthcan be suppressed short by using the dispersion compensator 66 formedfrom a DCF, the loss of the dispersion compensator 66 can be suppressedlow. Particularly where each of the optical fibers 4, 8 and 12 has anormal dispersion, a 1.3 μm zero dispersion fiber is suitable for thedispersion compensator 66. For example, where a plurality of suchdispersion compensators 66 are provided intermediately of the opticalfiber 8, the dispersion compensators 66 are preferably provided at equalintervals in the longitudinal direction of the optical fiber 8.

[0148] While, in FIG. 12, the dispersion compensator 66 is added to thebasic construction of FIG. 1, at least one dispersion compensator may beprovided additionally in the first to third embodiments of the presentinvention.

[0149] Referring to FIG. 13, there is shown a construction of adispersion compensator which uses a fiber grating FG. The dispersioncompensator can be used as the dispersion compensator 66 of FIG. 12 orfor an application which will be hereinafter described. Optical pulseswhose wavelengths of both edges are λ₁ and λ₂ are supplied to the fibergrating FG through an optical circulator OC. The grating pitch of thefiber grating FG has a predetermined distribution, and the beam of thewavelength λ₁ is Bragg reflected at a position comparatively near to theoptical circulator OC, but the beam of the wavelength λ₂ is Braggreflected at another position comparatively far from the opticalcirculator OC. Consequently, compression of the optical pulses isperformed, and dispersion compensation can be performed by extracting aBragg reflected beam from the fiber grating FG through the opticalcirculator OC.

[0150] Referring to FIG. 14, there is shown a fifth embodiment of thepresent invention. Here, a system is shown which further includes, incontrast with the basic construction of FIG. 1, an optical unit 68 whichincludes optical elements which individually correspond to the firstoptical fiber 4, first phase conjugator 6, second optical fiber 8,second phase conjugator 10 and third optical fiber 12. A first end ofthe optical unit 68 is connected to the third optical fiber 12 at apoint A which corresponds to the optical receiver 14 of FIG. 1, and asecond end of the optical unit 68 is connected to an optical receiver14′. The optical unit 68 includes an optical fiber 4′, a phaseconjugator 6′, an optical fiber 8′, a phase conjugator 10′and an opticalfiber 12′ which correspond to the optical fiber 4, phase conjugator 6,optical fiber 8, phase conjugator 10 and optical fiber 12, respectively.The optical unit 68 has a system midpoint 16′ which corresponds to thesystem midpoint 16 of FIG. 1. While, in the embodiment of FIG. 14, onlyone optical unit 68 is shown, a plurality of optical units 68 may beprovided in series between the point A and the optical receiver 14′.

[0151] With the present embodiment, by applying the conditions of thepresent invention to individual portions of the system shown, thedistance between the optical transmitter 2 and the optical receiver 14′can be increased sufficiently. Further, since the waveform of an opticalsignal restores its original waveform at the system midpoints 16 and 16′and the point A, adding/dropping of an optical signal or monitoring ofan optical signal waveform can be performed readily by providing a nodeat each of the points. Further, by applying the optical band-pass filter56 of the optical amplifier of FIG. 9 to at least one of the systemmidpoints 16 and 16′ and the point A, noise by ASE can be removedefficiently.

[0152] It is to be noted that, while the fifth embodiment here isdescribed in contrast with the basic construction of FIG. 1, the firstembodiment of FIG. 7 may be applied to the fifth embodiment.

[0153] Referring to FIG. 15, there is shown a sixth embodiment of thepresent invention. In the third embodiment of FIG. 11, in order to applythe basic construction of FIG. 1 to WDM (wavelength division multiplex),a plurality of first optical fibers 4-1, . . . , n are providedcorresponding to a plurality of optical transmitters 2-1, . . . , n anda plurality of third optical fibers 12-1, . . . , n are providedcorresponding to a plurality of third optical receiver 14-1, . . . , n.In contrast, in the sixth embodiment of FIG. 15, an optical multiplexer62′ is provided directly after the optical transmitters 2-1, . . . , nand a common first optical fiber 4 is provided between the opticalmultiplexer 62′ and the first phase conjugator 6. Further, an opticaldemultiplexer 64′ is provided immediately before the optical receivers14-1, . . . , n and a common third optical fiber 12 is provided betweenthe second phase conjugator 10 and the optical demultiplexer 64′.

[0154] The wavelengths of signal beams outputted from the opticaltransmitters 2-1, . . . , n are different from each other. Accordingly,if the wavelength channel regarding the optical transmitter 2-1 and theoptical receiver 14-1 satisfies the conditions of the expressions (1) to(4), then the waveform regarding the wavelength channel restores itsoriginal waveform fully at the system midpoint 16, but, since, in astrict sense, the expressions (1) to (4) cannot be satisfied regardingthe other wavelength channels, the waveforms of the wavelength channelsmay not restore their original waveforms fully at the system midpoint16. However, in the present invention, by performing signal settingsymmetrical with respect to the system midpoint 16, with regard to anywavelength channel whose waveform does not restore its original waveformfully at the system midpoint 16, the waveform can restore its originalwaveform fully on the reception side.

[0155] Referring to FIG. 16, there is shown a seventh embodiment of thepresent invention. Here, the second portion 82 of the second opticalfiber 8, the phase conjugator 10 and the third optical fiber 12 of FIG.15 are omitted, and an optical demultiplexer 64″ is provided at thesystem midpoint 16. A design example of dispersion parameters where thephase conjugator 6 of FIG. 16 has a third-order nonlinear optical mediumis described.

[0156] As seen from FIG. 17A, where the wavelengths of signal beamsoutputted from the optical transmitters 2-1, . . . , n are representedby λ_(s1), . . . , λ_(sn), respectively, the wavelength λ_(c1), . . . ,λ_(cn) of phase conjugate beams outputted from the phase conjugator 6are disposed at positions symmetrical with the wavelengths λ_(s1), . . ., λ_(sn) A of the signal beams with respect to the wavelength λ_(P) ofpump light. If it is assumed that, in the system of FIG. 16, for theoptical fiber 4 and the first portion 81 before and after the phaseconjugator 6, optical fibers of a same type are used and the fibers havea characteristic that the dispersion parameter varies relying upon thewavelength as indicated by D₁ in FIG. 17A, then since the chromaticdispersions that the signal beams undergo are different for individualchannels, the compensation may become incomplete. Therefore, in theexample shown in FIG. 17A, where a fiber having such a characteristic asindicated by D₁ is used for the first optical fiber 4 in which signalbeams of the wavelengths λ_(s1), . . . , λ_(sn) propagate, a fiberhaving such a characteristic symmetrical with D₁ with respect to thewavelength λ_(P) of the pump light as indicated by D₂ is used for thefirst portion 81 in which phase conjugate beams of the wavelengthsλ_(c1), . . . , λ_(cn) propagate. For example, where the dispersiongradient (second-order dispersion; wavelength differentiation of adispersion parameter) of the first optical fiber 4 is in the positive,the dispersion gradient of the first portion 81 is set to the negative.By making the chromatic dispersion that a signal beam of each channelundergoes and the chromatic dispersion that a corresponding phaseconjugate beam undergoes equal to each other in this manner, thechromatic dispersion and the nonlinearity can be compensated for foreach channel.

[0157] Particularly where WDM is applied, although waveformdeterioration is caused not only by SPM which occurs with each channelbut also by XPM (cross phase modulation) by a mutual action betweenchannels, the XPM can be compensated for by designing the dispersionparameters in such a manner as illustrated in FIG. 17A. It is to benoted that, where a DD-DCF is applied to the embodiment of FIG. 16, forexample, a DD-DCF having a characteristic of that of FIG. 17A shifted ina vertical direction can be used for each of the DD-DCFs.

[0158] Also fibers having no dispersion gradient as seen in FIG. 17B canbe used. In particular, before and after the first phase conjugator 6, afiber whose dispersion parameter D1 does not vary in accordance with thewavelength is used for the optical fiber 4 while another fiber whosedispersion parameter D₂ does not vary in accordance with the wavelengthis used for the first portion 81. By using fibers which do not have adispersion gradient in this manner, not only wavelength deterioration bySPM and XPM but also FWM between channels can be compensated for. Sincethe occurrence efficiency of FWM relies upon the dispersion value ofeach fiber, it is desirable to make the dispersion parameters of thefirst optical fiber 4 and the first portion 81 equal to each other. Itis to be noted that, since the occurrence efficiency of FWM betweenchannels has a polarization dependency, where WDM is applied as seen inFIG. 16, it is preferable to provide a polarizing scrambler immediatelyafter each of the optical transmitters 2-1, . . . , n or immediatelyafter the optical multiplexer 62′. Further, the DD-DCFs may beconstructed using fibers having such a characteristic as seen in FIG.17B.

[0159] Referring to FIGS. 18A and 18B, there is shown an optical networkto which the present invention can be applied. In the optical networkshown in FIG. 18A, three terminal stations 70 are connected to eachother by optical fibers, and a node 72 for adding/dropping an opticalsignal is provided intermediately of each of the optical fibers. Each ofthe terminal stations 70 has the phase conjugator 6 or 10 of FIG. 1 sothat the conditions of the present invention may be satisfied for eachof the optical fibers. Each of the nodes 72 is disposed at the systemmidpoint 16 (refer to FIG. 1) according to the present invention. Sincethe waveform restores its original waveform at each system midpoint, bydisposing the nodes 72 at the system midpoints, adding/dropping of anoptical signal is allowed without taking deterioration of the waveforminto consideration.

[0160] Where WDM is applied to the optical network of FIG. 18A, each ofthe terminal stations 70 preferably has a phase conjugator for eachchannel of WDM. Where each of the terminal stations 70 has a pluralityof phase conjugators in this manner, when an optical signal passes eachterminal station 70, phase conjugate conversion and wavelengthconversion are performed for each channel, branching or change-over(cross connection) of an optical signal can be performed by each of theterminal stations 70.

[0161] In the optical network shown in FIG. 18B, two terminal stations70 are disposed on a main line, and a node 72 is provided between theterminal stations 70. Each of the terminal stations 70 is connected to asub system 74. Each of the sub systems 74 has a ring-like optical fibernetwork and a plurality of nodes 76 provided intermediately of theoptical fiber network.

[0162] With the construction described above, for example, where WDM isapplied to the main line network, by allocating channels of WDMindividually to the sub systems 74, a comparatively low speed LAN (localarea network) can be provided readily.

[0163] Referring to FIG. 19, there is shown another basic constructionof an optical communication system according to the present invention.The present system includes an optical transmitter 102 for outputting asignal beam, a first optical fiber 104 for transmitting the signal beam,a phase conjugator 106 for converting the signal beam transmitted by thefirst optical fiber 104 into a phase conjugate beam and outputting thephase conjugate beam, a second optical fiber 108 for transmitting thephase conjugate beam, and an optical receiver 110 for receiving thephase conjugate light transmitted by the second optical fiber 108.

[0164] On an optical path which includes the first optical fiber 104,phase conjugator 106 and second optical fiber 108, at least onedispersion compensator 112 for providing chromatic dispersions of theopposite signs to those of the chromatic dispersions of the opticalfibers 104 and 108. While, in the example shown, the dispersioncompensator 112 is provided intermediately of the second optical fiber108, the dispersion compensator 112 may be provided intermediately ofthe first optical fiber 104. Further, the dispersion compensator 112 maybe provided at an end portion of the optical fiber 104 or 108.

[0165] Parameter setting of the first optical fiber 104 and the secondoptical fiber 108 is performed in conformity with parameter setting ofthe first optical fiber 4 and the first portion 81 of the second opticalfiber 8 of the system of FIG. 1. For example, the product of the averagevalue of the chromatic dispersion and the length of the first opticalfiber 104 is set substantially equal to the product of the average valueof the chromatic dispersion and the length of the second optical fiber108. When to calculate the average value of each chromatic dispersion,the dispersion value of the dispersion compensator 112 may or may not beincluded.

[0166] When the system of FIG. 19 is worked, there is a demand to use asingle mode fiber which provides the lowest loss and anomalousdispersion in a wavelength 1.55 μm band for the optical fiber 104 or108. The reason is based on the fact that, firstly, optical fibertransmission lines formed from such single mode fibers are already laidin many regions and it is desirable to utilize them as they are and thefact that, secondly, when WDM is performed in the wavelength 1.55 μmband, a comparatively large anomalous dispersion occurs with a singlemode fiber and consequently crosstalk between channels by XPM and FWMoccurs less likely.

[0167] Where the dispersion compensator 112 is not present, if it istried to provide the optical fiber 104 in a terminal station and use theoptical fiber 108 as a transmission line, since the optical fiber 104must be made comparatively short, for example, where the dispersionparameter of the optical fiber 108 is +18 ps/km/nm, the dispersionparameter of the optical fiber 104 must be set to a value higher thanthis value. However, since it is difficult in the existing circumstancesto acquire an optical fiber which provides such a high anomalousdispersion, the system is limited. In contrast, since the totaldispersion of the optical fiber 108 can be made low by using thedispersion compensator 112 as shown in FIG. 19, it is possible to usethe optical fiber 104 which has a dispersion parameter equal to thedispersion parameter of the optical fiber 108.

[0168] While, in the example shown, the single dispersion compensator112 is provided, setting of the conditions of the present invention canbe performed readily by disposing a plurality of dispersioncompensators, for example, uniformly in the longitudinal direction.

[0169] It is to be noted that, where the optical fibers 104 and 108 areeach formed from a single mode fiber which provides an anomalousdispersion, an optical fiber which provides a normal dispersion can beused for the dispersion compensator 112. Further, a dispersioncompensator which employs a fiber grating described hereinabove withreference to FIG. 13 may be used.

[0170] In the following, additional description of the present inventionis provided. When the present invention is worked, most simply the totaldispersions and/or the total nonlinear effects before and after a phaseconjugator are set equal to each other as apparently seen from FIG. 2and the expressions (22) and (23). While, in the expressions (22) and(23), D_(j) and γ_(j) (j=1, 2) are handled as constants, since, inactual parameter setting, the dispersion value and the nonlinearcoefficient exhibit different values depending upon the position of thefiber, in order to anticipate accuracy, average values of them areadopted.

[0171] Compensation in accordance with the expressions (22) and (23) isapproximation which stands when the nonlinear effect is not very high.More particularly, the compensation is approximation effective where thelength of the optical fiber or the repeating distance by opticalamplifiers is sufficiently shorter than the nonlinear length of theoptical fiber. For example, if a case is considered wherein signal lightof an average peak power +5 dBm is transmitted by ordinary DSFs(dispersion shift fibers) whose nonlinear coefficient is 2.6 W⁻¹km⁻¹,then the nonlinear length is 121.6 km. Accordingly, if the optical fiberlength or the repeating distance by optical amplifiers is shorter thanapproximately 100 km, then the chromatic dispersion and the nonlineareffect can be compensated for by the approximation described above.

[0172] However, if the power becomes further higher, then thecompensation comes to a limit due to the asymmetry of the optical powerdistribution before and after the phase conjugator by the loss of theoptical fiber. In such a case, the waveform distortion by the chromaticdispersion and the nonlinear effect can be compensated for by satisfyingthe conditions of the expressions (20) and (21) in accordance with thepresent invention.

[0173] Generally, since a transmission line exhibits a loss, in order tosatisfy the expressions (20) and (21), some loss compensation effectmust be provided. Several methods may be possible for this. The firstmethod is to use a gain medium of the distributed constant type for thetransmission line. A Raman amplifier, an amplifier of the in-line typewhich employs an EDF and so forth may be used. The second method is tocontrol the ratio between the nonlinear effect and the dispersion value.In order to compensate for a decrease of the nonlinear effect along thetransmission line by a loss, either the dispersion should be decreasedor the nonlinear effect should be increased along the transmission line.To vary the value of the dispersion is possible and promising by adesign of the optical fiber. The value of the dispersion can,be varied,for example, by varying the zero dispersion wavelength of the dispersionshift fiber (DSF) or by varying the difference in specific refractiveindex between the core and the clad of the fiber or the core diameter ofthe fiber. Meanwhile, to vary the nonlinear effect is possible byvarying the nonlinear refractive index or by varying the optical power.

[0174] In order to increase the optical intensity along a transmissionline which exhibits a loss, the effective core sectional area A_(eff) ofthe fiber should be decreased gradually along the longitudinal directionof the fiber. For example, if the mode field diameter (MFD) decreases toone half, then the optical intensity increases to approximately fourtimes. Accordingly, the loss of approximately 6 dB can be compensatedfor only by this. For a higher loss, the MFD must be further reduced,but if the MFD becomes too small, then the loss is increased thereby andthis provides a contrary effect. A realistic minimum value of the MFD isestimated to be approximately 3 μm. If it is taken into considerationthat the MFD of a 1.3 μm zero dispersion SMF (single mode fiber) isapproximately 10 μm and the MFD of a 1.55 μm zero dispersion DSF(dispersion shifted fiber) is approximately 8 μm, then the loss whichcan be dealt with only by the MFD is approximately 10 dB for the SMF andapproximately 8 dB for the DSF.

[0175] Where a further higher loss is involved, it is a possible idea todecrease the MFD and decrease the value of the dispersion. For example,if the value of the dispersion can be reduced to one half, then evenwhere a further loss of 3 dB is involved, the ratios between thedispersion and the nonlinear effect can be made symmetrical with respectto the phase conjugator. With a dispersion compensation fiber (DCF)development of which has been proceeded in recent years, the dispersionvalue can be varied within a range from approximately −120 ps/nm/km toapproximately −10 ps/nm/km, and besides, it is also possible to make theMFD lower than 5 μm. Accordingly, compensation for the loss ofapproximately 10 dB is possible by connecting a plurality of DCFs havingdifferent dispersion values from each other in cascade connection, forexample, by splicing.

[0176] If it is assumed that the average dispersion of a transmissionline (for example, the first portion 81 of the optical fiber 8 of FIG.7) is −0.5 ps/nm/km, then if the average dispersion of a compensationfiber (for example, the first optical fiber 4 of FIG. 7) is set to −50ps/nm/km, then a system can be constructed using a compensation fiber ofa length equal to {fraction (1/100)} that of the transmission line. Inthis instance, if the loss of the compensation fiber is, for example,0.4 dB/km, then the compensation conditions can be realized bydecreasing the absolute value of the dispersion value at the rate of 0.4dB/km. If the overall length of the transmission line is 2,000 km, thena compensation fiber of 20 km is used, and the difference in dispersionvalue in this instance is 8 dB. It is to be noted that, while also theoptical intensity in the compensation fiber must be set to approximately100 times the optical intensity of the transmission line, if the MFD ofthe compensation fiber is, for example, 4 μm, then the optical power maybe required to be only approximately 25 times.

[0177] In long-haul transmission wherein an optical amplifier is used,it is known that it is desirable to use a normal dispersion fiber for atransmission line in order to reduce the nonlinear distortion by noiselight of the optical amplifier. Accordingly, a system construction forwhich a DCF described above is used is promising.

[0178] In the phase conjugator of FIG. 6, if the power of signal lightor pump light supplied to the optical fiber 18 which is used as anonlinear optical medium or phase conjugate light produced in theoptical fiber 18 exceeds a threshold value of stimulated Brillouinscattering (SBS) of the optical fiber 18, then the conversion efficiencyfrom the signal light into the phase conjugate light becomes low. Inorder to suppress the influence of the SBS, frequency modulation orphase modulation may be applied to at least one of the signal light andthe pump light. The modulation rate in this instance is sufficient withapproximately several hundreds kHz, and since this modulation rate isgenerally sufficiently lower than the modulation rate for signal light,there is no possibility that the transmission characteristic may bedeteriorated by modulation for suppression of the SBS.

[0179] Since the nonlinear coefficient γ of an ordinary DSF (dispersionshifted fiber) is as low as approximately 2.6 W⁻¹km⁻¹, in order toobtain a sufficient conversion efficiency where an ordinary DSF is usedas a nonlinear optical medium for generating phase conjugate light suchas, for example, the optical fiber 18 of FIG. 6, it is demanded to makethe fiber length longer than 10 km. Accordingly, it is demanded toprovide a DSF having a nonlinear coefficient γ sufficiently high to makethe fiber length short. If the length of a DSF which is used as anonlinear optical medium for generating phase conjugate light can bemade short, then the zero dispersion wavelength can be managed with ahigh degree of accuracy, and accordingly, it becomes easy to make thewavelength of pump light coincide accurately with the zero dispersionwavelength of the DSF. As a result, a broad conversion band can beobtained. Here, the conversion band is defined as a maximum detuningwavelength (detuning frequency) of pump light and signal light in thecondition that phase conjugate light of a certain power is obtained.

[0180] In order to increase the nonlinear coefficient γ defined by theexpression (6), it is effective to increase the nonlinear refractiveindex n₂ or decrease the mode field diameter (MFD) which corresponds tothe effective core sectional area A_(eff). In order to increase thenonlinear refractive index n₂, for example, the clad should be dopedwith fluorine or a like element while the core should be doped with GeO₂of a high density. By doping the core with GeO₂ by 25 to 30 mol %, ahigh value of 5×10⁻²⁰ m²/W or more is obtained (with an ordinary silicafiber, approximately 3.2×10⁻²⁰ m²/W). To decrease the MFD is possible bya design of the specific refractive index difference Δ or the shape ofthe core. Such a design of the DSF is similar to that of the DCF(dispersion compensation fiber). For example, by doping the core withGeO₂ by 25 to 30 mol % and setting the specific refractive indexdifference Δ to 2.5 to 3.0%, a value of the MFD lower than 4 μm has beenobtained. As a synthetic effect of them, a value of the nonlinearcoefficient γ higher than 15 W⁻¹km⁻¹ has been obtained.

[0181] As another important factor, it is listed that a DSF whichprovides a nonlinear coefficient γ having such a high value as mentionedabove should have a zero dispersion wavelength included in a pump band.Such coincidence between the zero dispersion wavelength and the pumpband is possible by setting fiber parameters (for example, the specificrefractive index difference Δ and the MFD) in the following manner. Inan ordinary optical fiber, as the specific refractive index difference Δincreases in a condition that the MFD is kept fixed, the dispersionvalue increases in a normal dispersion region. Such a DD-DCF which isused for pre-compensation or post-compensation using a phase conjugatoras described above is realized based on such a principle as justdescribed. Meanwhile, if the core diameter increases, then thedispersion decreases, but if the core diameter decreases, then thedispersion increases. Accordingly, a zero dispersion with respect topump light can be obtained by first setting the MFD to a certain valuewhich conforms with the pump band and then adjusting the core diameterso that the zero dispersion wavelength may coincide with a value of thepump light set in advance.

[0182] The conversion efficiency ηc of an optical fiber of a loss α canbe approximated by

ηc=exp(−αL)(γP _(p) L)²  (24)

[0183] where P_(p) is the average pump light power. Accordingly, a fiberwhose nonlinear coefficient γ is 15 W³¹ ¹km⁻¹ can achieve an equalconversion efficiency with a length of approximately 2.6/15≈1/5.7comparing with an ordinary DSF. While an ordinary DSF requires a lengthof approximately 10 km as described above in order to obtain asufficient conversion efficiency, a fiber having such a high nonlinearcoefficient γ as described above can achieve a similar conversionefficiency with a length of approximately 1 to 2 km. Actually, since theloss decreases as the fiber length decreases, the fiber length can befurther decreased in order to obtain an equal conversion efficiency.With a DSF of such a small length, the controllability of the zerodispersion wavelength is augmented, and accordingly, the wavelength ofthe pump light can be made accurately coincide with the zero dispersionwavelength and a broad conversion band can be obtained. Further, wherethe fiber length is several km, since the polarization plane maintainingcapacity is secured, use of such a DSF as described above is veryeffective to achieve a high conversion efficiency and a broad conversionband and eliminate the polarization dependency.

[0184] In order to make four wave mixing occur effectively using anoptical fiber, it is important to match the phases of pump light andphase conjugate light. The phase mismatching amount Δk is approximatedin the following manner:

Δk=δω²β₂(ω_(p))+2γP _(p)  (25)

[0185] where β₂(ω_(p)) is the chromatic dispersion at the pump lightfrequency ω_(p), and δω is the frequency difference between the signallight and the pump light. Unless pump light of a particularly high power(for example, 100 mW or more) is used, the second term of the expression(25) is sufficiently smaller than the first term, and accordingly, thesecond term can be ignored. Accordingly, the phase matching (to cause Δkto approach 0 infinitely) can be achieved by making the wavelength ofthe pump light coincide with the zero dispersion wavelength of thefiber. However, in an actual fiber, since the zero dispersion wavelengthfluctuates in the longitudinal direction, it is not easy to maintain thephase matching condition over the overall length of the fiber.

[0186] In this manner, in an apparatus which includes an optical fiberas a nonlinear optical medium for generating phase conjugate light, theconversion band is limited by the dispersion of the optical fiber.Accordingly, if an optical fiber is produced wherein the dispersionthereof in the longitudinal direction is controlled fully so that theoptical fiber has, for example, a single zero dispersion wavelength overthe overall length (accurately, the nonlinear length), then a conversionband which is infinitely great in fact (so broad that there is no limitwithin a range within which the dispersion gradient exhibits a straightline) is obtained by adjusting the pump light wavelength to the zerodispersion wavelength. Actually, however, since the zero dispersionwavelength fluctuates along the longitudinal direction because of aproblem in a technique of production of an optical fiber, the phasematching condition is displaced from its ideal condition, and theconversion band is limited thereby.

[0187] However, even in such a case as just described, by cutting anoptical fiber to divide it into a plurality of small intervals (orsections) and joining each two intervals which have similar zerodispersion wavelengths to each other by splicing or the like (in anorder different from the initial order as counted from an end of thefiber), an optical fiber suitable for provision of a phase conjugatorwhich has a broad conversion band although the average dispersion overthe overall length is equal can be obtained.

[0188] Or else, a large number of fibers of a length (for example,several hundreds m or less) with which dispersion control is possiblewith such a degree as is necessary to obtain a sufficiently broadconversion band are prepared in advance, and those fibers which haverequired zero dispersion wavelengths are spliced in combination toobtain a fiber of a length necessary to obtain a required conversionefficiency. Then, by providing a phase conjugator using the fiber, abroad conversion band can be obtained.

[0189] Where the conversion band is expanded in this manner, since thepower of the pump light is high in the proximity of the pump light inputend of the nonlinear optical medium, it is effective to gather thoseportions which have shorter zero dispersion wavelengths or thoseportions whose fluctuations in zero dispersion wavelength are smaller inthe proximity of the pump light input end. Further, by successivelyincreasing the dividing number in accordance with the necessity or, at aposition which is spaced away from the pump light input end and at whichthe dispersion value is comparatively high, by suitably combining thefibers by disposing them such that the positive and negative signs ofthe dispersion values appear alternately or the like, the conversionband can be further increased.

[0190] In order to determine, when an optical fiber is to be divided, towhich degree each section should be made short to achieve a sufficienteffect, for example, the nonlinear length should be used as a standard.Similarly as in compensation for the nonlinear effect, in FWM (four wavemixing) in a fiber which is sufficiently short comparing with thenonlinear length, it can be considered that the phase matching reliesupon the average distribution value of the fiber. As an example, in FWMwherein a fiber whose nonlinear coefficient γ is 2.6 W⁻¹km⁻¹ and pumplight power of approximately 30 mW is used, since the nonlinear lengthis approximately 12.8 km, approximately one tenth the length, that is,approximately 1 km, is considered as a standard. As another example, inFWM wherein a fiber whose nonlinear coefficient γ is 15 W⁻¹km⁻¹, andpump light power of approximately 30 mW is used, since the nonlinearlength is approximately 2.2 km, one tenth the length, that is, 200 m,may be considered as a standard. Anyway, if the average zero dispersionwavelengths of fibers which are sufficiently short comparing with thenonlinear lengths are measured and those fibers which have substantiallyequal values are combined to provide a nonlinear optical medium having arequired conversion efficiency, then a phase conjugator of a broadconversion band can be obtained.

[0191] In this manner, according to the present invention, a firstmethod for producing an apparatus which has a nonlinear optical mediumfor generating phase conjugate light is provided. In this method, anonlinear optical medium is provided by cutting an optical fiber into aplurality of intervals first, and then re-arranging and joining togethera plurality of ones of the intervals so that the conversion band innondegenerative four wave mixing in which the nonlinear optical mediumis used may be maximum. Phase conjugate light is generated by supplyingpump light and signal light to the nonlinear optical medium. Since theconversion band from the signal light to the phase conjugate light issufficiently broad, for example, where WDM signal light obtained bywavelength division multiplexing a plurality of optical signals havingdifferent wavelengths from each other is used as the signal light, theplurality of optical signals are collectively converted into phaseconjugate light (a plurality of phase conjugate light signals).

[0192] Preferably, the dispersion value (for example, the dispersionvalue with regard to pump light) of each of the plurality of intervalsis measured, and the plurality of intervals are re-arranged so thatthose intervals which have comparatively low dispersion values aredisposed on the side nearer to an input end when the pump light isinputted to the nonlinear optical medium. Consequently, since the phasematching conditions can be obtained effectively at a portion where thepower of the pump light is high, the conversion band is expandedeffectively.

[0193] Preferably, at least some of the plurality of intervals arejoined together such that the positive and negative signs of thedispersion values appear alternately. Consequently, since the averagedispersion of each portion of the optical fiber can be suppressed low,effective expansion of the conversion band can be achieved.

[0194] Further, according to the present invention, a second method forproducing an apparatus which includes a nonlinear optical medium forgenerating phase conjugate light is provided. In this method, anonlinear optical medium is obtained by cutting an optical fiber into aplurality of intervals first, then measuring the dispersion value (forexample, the dispersion value with regard to pump light) of each of theplurality of intervals and then selecting and joining together onlythose of the intervals which have dispersion values sufficiently low toobtain a required conversion band by nondegenerative four wave mixing inwhich the nonlinear optical medium is used. Also where a phaseconjugator is implemented using the nonlinear optical medium obtained bythe present second method, since a broad conversion band is obtained,collective conversion of WDM signal light is possible.

[0195] While, in each of the first and second methods according to thepresent invention, an optical fiber is first cut into a plurality ofintervals, the present invention is not limited to this. For example, anoptical fiber may be cut in the following manner in accordance with thenecessity.

[0196] In particular, according to the present invention, a third methodfor producing an apparatus which includes a nonlinear optical medium forgenerating phase conjugate light is provided. In the present method, thedeviation of the zero dispersion wavelength of an optical fiber ismeasured, and then, if the measured deviation exceeds a range determinedin advance, then the optical fiber is cut so that the resulting fibersmay have deviations in zero dispersion wavelength which remain withinthe range, whereafter the optical fiber or the cut fibers each having azero dispersion wavelength substantially equal to the wavelength of pumplight are selected and the selected fibers are joined together to obtaina nonlinear optical medium.

[0197] Measurement of a deviation of the zero dispersion wavelength canbe performed using, for example, that the generation efficiency of fourwave mixing is different in accordance with the zero dispersionwavelength. While generally a chromatic dispersion can be determined bymeasuring a wavelength dependency of the group velocity, since the phasematching in four wave mixing exhibits the best condition when the pumplight wavelength and the zero dispersion wavelength coincide with eachother as described hereinabove, the zero dispersion wavelength can bedetermined as a pump light wavelength which provides a maximumoccurrence efficiency by measuring generation efficiencies of four wavemixing (phase conjugate light) with respect to pump light wavelengths ina condition wherein the wavelength difference between pump light andsignal light is kept to a comparatively high fixed value of, forexample, approximately 10 to 20 nm. Further, the generation efficiencyof four wave mixing increases in proportion to the square of theintensity of pump light. Accordingly, when the zero dispersionwavelength exhibits a variation in the longitudinal direction of theoptical fiber, zero dispersion wavelengths which are different betweenwhere signal light and pump light are inputted from one end face of anoptical fiber and where signal light and pump light are inputted fromthe other end face are measured usually. Accordingly, a deviation inzero dispersion wavelength of the optical fiber can be determined basedon the two measurement values of the zero dispersion wavelength. This isdescribed more particularly.

[0198] Referring to FIG. 20, there is illustrated a production process120 for a nonlinear optical medium wherein the deviation of the zerodispersion wavelength is small. In step 122, the allowable range Δλ₀ ofthe zero dispersion wavelength is determined. The range Δλ₀ can bedetermined as a requested characteristic of a system from a requiredconversion band, and a concrete value of it is, for example, 2 nm. Then,in step 124, the deviation δλ of the zero dispersion wavelength ismeasured. For example, if an optical fiber F1 is given, then the zerodispersion wavelength λ₀₁ obtained when signal light and pump light areinputted from a first end of the optical fiber Fl and the zerodispersion wavelength λ₀₂ obtained when signal light and pump light areinputted from a second end of the optical fiber F1 are measureddepending upon the generation efficiency of four wave mixing describedabove. In this instance, |λ₀₁−λ₀₂| can be used as an alternate value ofthe deviation δλ of the zero dispersion wavelength.

[0199] Then in step 126, it is discriminated whether or not thedeviation δλ is smaller than the range Δλ₀. Here, the succeeding flow isdescribed under the assumption that δλ≧Δλ₀. In step 128, the opticalfiber Fl is divided into two optical fibers F1A and F1B by cutting.After step 128, the control returns to step 124, in which the deviationδλ is measured for each of the optical fibers F1A and F1B anddiscrimination is performed for each measurement value in step 126.Here, if it is assumed that each deviation δλ is smaller than Δλ₀, thenthe flow comes to an end. It is to be noted that the cutting point ofthe optical fiber F1 in step 128 is arbitrary, and accordingly, thelengths of the optical fibers F1A and F1B may be equal to each other ormay be different from each other.

[0200] While, in the description above, steps 124 and 126 are repeated,steps 124 and 126 may not be repeated or may be repeated by a greaternumber of times. For example, if an optical fiber F2 wherein thedeviation of the zero dispersion frequency is small is given, then thecondition is satisfied in the first time discrimination of step 126, andin this instance, the optical fiber F2 is not cut. On the other hand, ifan optical fiber F3 whose zero dispersion wavelength exhibits a greatvariation in the longitudinal direction, then the optical fiber F3 isdivided into optical fibers F3A and F3B in step 128 for the first time,and then, if it is discriminated in the second discrimination step 126that the optical fiber F3A satisfies the condition but the optical fiberF3B does not satisfy the condition, then the optical fiber F3B may bedivided into optical fibers F3B1 and F3B2 in step 128 for the secondtime and the flow may come to an end. In this instance, the threeoptical fibers F3A, F3B1 and F3B2 are obtained from the original opticalfiber F3, and the deviation of the zero dispersion wavelength of eachfiber is smaller than the allowable range αλ₀.

[0201] A plurality of optical fiber pieces (F1A, F1B, . . . ) obtainedin this manner are put in order for different values of the zerodispersion wavelength, and those optical fiber pieces having zerodispersion wavelengths substantially equal to the wavelength of pumplight for four wave mixing are selected and joined together until alength with which a required conversion coefficient can be obtained isreached. By this, a nonlinear optical medium wherein the deviation ofthe zero dispersion wavelength in the longitudinal direction is verysmall can be obtained. By implementing a phase conjugator using thisnonlinear optical medium, a broad conversion band can be obtained.

[0202] Even if the values of the zero dispersion wavelengths λ₀₁ and λ₀₂are substantially coincident with each other, also an optical fiberwherein the zero dispersion wavelength exhibits a large deviation in thelongitudinal direction is presumable. This is a case wherein, forexample, the distribution of the zero dispersion wavelength in thelongitudinal direction is symmetrical with respect to the center of theoptical fiber in the longitudinal direction. In such an instance, priorto the process 120, division of the optical fiber at least into twooptical fiber pieces is performed, and then the process 120 should beapplied to each of the optical fiber pieces. Or, the process 120 may berepeated by a plurality of numbers of times.

[0203] From an experiment, it has become apparent that a phaseconjugator implemented using a nonlinear optical medium obtained by themethod of the present invention has a conversion band broader than 40 nmfor a signal of 10 Gb/s. This phase conjugator has a substantially fixedvalue −10.9 dB as a conversion efficiency under the pump light power of+15 dBm without depending upon the detuning wavelength within a range ofthe detuning wavelength between signal light and pump light whichexceeds 21 nm. In particular, the conversion band is broader than 40 nm.This nonlinear optical medium particularly is a highly nonlineardispersion-shifted fiber (HNL-DSF: highly nonlinear dispersion-shiftedfiber) of 750 m. The HNL-DSF was obtained by splicing three intervalsindividually of 250 m. The average zero dispersion wavelengths of theindividual intervals were 1,547.3 nm, 1,546.3 nm and 1,548.4 nm,respectively. The average zero dispersion wavelength of the HNL-DSF as aresult was measured to be 1,547.2 nm. The MFD (mode field diameter) was3.8 μm, the nonlinear coefficient γ was 20.4 W⁻¹km⁻¹, and the dispersiongradient was 0.032 ps/nm²/km.

[0204] In this manner, by using an optical fiber having a high nonlinearcoefficient and applying the present invention to restrict the deviationof the zero dispersion wavelength substantially within ±1 nm, provisionof a phase conjugator which exhibits a high conversion efficiency andhas a broad conversion band is possible. If it is taken intoconsideration that the conversion band of a conventional phaseconjugator which has an optical fiber as a nonlinear optical medium isseveral nm to the utmost, then advantages achieved by the method of thepresent invention are not self-evident from or are non-obvious orcritical over the prior art. Particularly where collective conversion ofWDM signal light is performed between optical networks using a phaseconjugator as in such an embodiment as hereinafter described, expansionof the conversion band by the present invention is very effective.

[0205] Nonlinear optical media obtained by the first, second and thirdmethods according to the present invention can be adopted for theoptical fiber 18 for the phase conjugator of FIG. 6. In this instance,since the coincidence between the wavelength of pump light outputtedfrom the laser diode 20 and the zero dispersion wavelength of theoptical fiber 18 can be maintained with a high degree of accuracy, abroad conversion band can be obtained.

[0206] Referring to FIG. 21, there is shown another construction exampleof the phase conjugator. The present phase conjugator uses an opticalfiber 18 similar to that in FIG. 6 as a nonlinear optical medium. Theoptical fiber 18 is preferably provided by the first, second or thirdmethod according to the present invention. Further, as a pump lightsource, a laser diode 20 is used. In order to guide signal light andpump light bidirectionally in the optical fiber 18 serving as anonlinear optical medium, an optical coupler 132 and a polarizing beamsplitter 134 are used. The optical coupler 132 has ports 132A, 132B and132C and outputs light supplied to the ports 132A and 132B from the port132C. An input port 130 is connected to the port 132A, and the port 132Bis connected to the laser diode 20 serving as a pump light source by anoptical fiber 133. The polarizing beam splitter 134 has ports 134A,134B, 134C and 134D. The ports 134A and 134B, and the ports 134C and134D, are coupled by a first polarization plane (for example, apolarization plane perpendicular to the plane of FIG. 21). The ports134A and 134C, and the ports 134B and 134D, are coupled by a secondpolarization plane (for example, a polarization plane parallel to theplane of FIG. 21) perpendicular to the first polarization plane. Theport 134A is connected to the port 132C by an optical fiber 135, and theoptical fiber 18 serving as a nonlinear optical medium is connectedbetween the ports 134B and 134C while the port 134D is connected to anoutput port 136. A polarization controller 138 which is formed in anordinary manner using a quarter-wave plate, a half-wave plate and soforth is provided intermediately of the optical fiber 18, and thepolarization controller 138 controls so that the polarization conditionsof an input and an output of the optical fiber 18 may coincide with eachother.

[0207] Signal light from the input port 130 and pump light from thelaser diode 20 are supplied to the port 134A of the polarizing beamsplitter 134 through the optical coupler 132. The signal light and thepump light thus supplied are separated into first and secondpolarization components having first and second polarization planes,respectively, by the polarizing beam splitter 134. The first and secondpolarization components propagate in the opposite directions to eachother in the optical fiber 18. In this instance, in the optical fiber18, two phase conjugate components which propagate in the oppositedirections to each other are generated by four wave mixing. Inparticular, the phase conjugate component which has the firstpolarization plane propagates from the port 134B toward the port 134C,but the phase conjugate component having the second polarization planepropagates from the port 134C toward the port 134B. The first and secondphase conjugate components supplied to the polarizing beam splitter 134are polarization combined, and the resultant phase conjugate light isoutputted from the port 134D toward the output port 136.

[0208] The polarization plane of the pump light outputted from the laserdiode 20 is preferably set such that the distribution ratio of the pumplight to the first and second polarization components separated by thepolarizing beam splitter 134 may be 1:1. For example, the laser diode 20is set so that the polarization plane of the pump light to be suppliedto the port 134A of the polarizing beam splitter 134 is inclined byapproximately 45 degrees with respect to both of the first and secondpolarization planes. As a result of such setting, since the twoorthogonal polarization components of the pump light act in coincidentpolarization planes on the two orthogonal polarization components of thesignal light which are guided in the opposite directions to each otherin the optical fiber 18, irrespective of the variation of thepolarization condition of the signal light at the input port 130, phaseconjugate light of a fixed intensity can be obtained. In other words,provision of a phase conjugator wherein the generation efficiency doesnot rely upon the polarization condition of input signal light ispossible.

[0209] In order that the polarization plane of pump light to be suppliedto the port 134A of the polarizing beam splitter 134 may be inclined byapproximately 45 degrees with respect to both of the first and secondpolarization planes, it is required to maintain the polarization planeof pump light, which is outputted as a substantially linearly polarizedwave from the laser diode 20, and supply the pump light to the port134A. To this end, a polarization maintaining fiber (PMF) can be usedfor each of the optical fibers 133 and 135. The PMF has a principal axisin a diametrical direction. The PMF maintains the polarization conditionof a polarization component having a polarization plane parallel to theprincipal axis or another polarization component having a polarizationplane perpendicular to the principal axis to propagate the polarizationcomponent. Accordingly, in order to make the polarization plane of pumplight inclined by 45 degrees with respect to the second polarizationplane at the port 134A, the principal axis of the PMF used as theoptical fiber 135 should be inclined by 45 degrees with respect to thefirst and second polarization planes.

[0210] However, where a PMF is used as the optical fiber 135, alsosignal light which is not necessarily limited to a linearly polarizedwave passes through the PMF, and polarization dispersion may be causedby a delay between two orthogonal polarization modes of the signal lightwhich passes through the PMF. In order to cope with the polarizationdispersion, the PMF to be used for the optical fiber 135 should beprovided by connecting first and second PMFs having substantially equallengths to each other by splicing. At the splicing connection point, thefirst principal axis of the first PMF and the second principal axis ofthe second PMF extend perpendicularly to each other, and consequently,the delay between the polarization modes is cancelled and thepolarization dispersion is eliminated. For example, where the firstprincipal axis is inclined by 45 degrees in the clockwise direction withrespect to the first polarization plane, the second principal axis isinclined by 45 degrees in the counterclockwise direction with respect tothe first polarization plane.

[0211] It is to be noted that, as described hereinabove, where thenonlinear coefficient of the optical fiber 18 is sufficiently high andthe length of it is so short that it has a polarization plane keepingcapacity, the polarization controller 138 can be omitted.

[0212] In this manner, according to the present invention, a phaseconjugator which has a generation efficiency which does not rely uponthe polarization condition of input signal light and has a lowpolarization dispersion is provided. This phase conjugator includes apolarization beam splitter, a nonlinear optical medium, a pump lightsource, and coupling means. The polarization beam splitter has first tofourth ports. The first and second ports, and the third and fourthports, are coupled to each other by a first polarization plane. Thefirst and third ports, and the second and fourth ports, are coupled toeach other by a second polarization plane perpendicular to the firstpolarization plane. The nonlinear optical medium is operativelyconnected between the second and third ports. The pump light sourceoutputs pump light. The coupling means includes an optical couplerhaving first and second input ports for receiving signal light and pumplight, respectively, and an output port, and a polarization maintainingfiber operatively connected between the output port and the first portof the polarization beam splitter. The coupling means supplies thesignal light and the pump light to the first port of the polarizationbeam splitter.

[0213] The polarization maintaining fiber has a principal axis set sothat the polarization plane of the pump light at the first port of thepolarization beam splitter is inclined substantially by 45 degrees withrespect to the first and second polarization planes.

[0214] Preferably, the polarization maintaining fiber includes first andsecond polarization maintaining fibers connected to each other bysplicing, and the first and second polarization maintaining fibers havefirst and second principal axes which extend perpendicularly to the eachother.

[0215]FIG. 22 is a view illustrating collective conversion of WDM signallight by a phase conjugator having a broad conversion band. WDM signallight is obtained by wavelength division multiplexing (WDM) opticalsignals of N channels having wavelengths λ₁, λ₂, . . . , λ_(N) differentfrom one another. Here, it is assumed that λ₁ is the shortest wavelengthand λ_(N) is the longest wavelength. The wavelength λ_(P) of the pumplight is set, for example, shorter than λ₁. The WDM signal light isconverted into converted light by nondegenerative four wave mixing usingthe pump light. The converted light includes converted optical signalsof N channels of wavelengths λ₁′, λ₂′, . . . , λ_(N)′, which aredifferent from one another. The arrangement of the optical signals ofthe individual channels of the WDM signal light and the convertedoptical signals of the converted light are symmetrical with respect tothe wavelength λ_(P) of the pump light.

[0216] In four wave mixing wherein an optical fiber is used as anonlinear optical medium, since the conversion band is substantiallyflat, wavelength conversion and phase conjugate conversion can beperformed in substantially equal conversion efficiencies for the opticalsignals of the individual channels. Accordingly, for each channel,waveform distortion by the chromatic dispersion and the nonlinear effectof the transmission line can be compensated for, and long-haullarge-capacity transmission is possible. While, in FIG. 22, conversionfrom a long wavelength band to a short wavelength band is illustrated,since the conversion band by an optical fiber is symmetrical withrespect to the zero dispersion wavelength, also conversion from a shortwavelength band to a long wavelength band can naturally be performed ina similar manner.

[0217]FIG. 23 is a block diagram showing an embodiment of a system towhich wavelength conversion and phase conjugate conversion are applied.A plurality of optical fiber networks NW1, NW2 and NW3 to each of whichWDM is applied are connected to each other by an optical fibertransmission line 140 and nodes 142. In order to perform conversionbetween the networks NW1 and NW2, a phase conjugator PC11 is providedintermediately of the optical fiber transmission line 140, and in orderto perform conversion between the optical fiber networks NW2 and NW3, aphase conjugator PC23 is provided intermediately of the optical fibertransmission line 140. It is assumed that, in the optical fiber networksNW1, NW2 and NW3, WDM transmission of wavelength bands λ_(1j), λ_(2j)and λ_(3j) different from each other is performed, respectively. Thephase conjugator PC11 performs wavelength conversion and phase conjugateconversion between the wavelength bands λ_(1j), and λ_(2j), and thephase conjugator PC23 performs wavelength conversion and phase conjugateconversion between the wavelength bands λ_(2j) and λ_(3j). Sincepositions at which the waveform distortion by the chromatic dispersionand the nonlinear effect is improved most in accordance with the presentinvention appear intermediately of the optical fiber transmission line140, the nodes 142 are provided individually at such positions. Each ofthe nodes 142 includes an optical adding/dropping apparatus forperforming addition and extraction of an optical signal. The opticaladding/dropping apparatus functions for all or some of channels of WDMsignal light or converted light. For example, if the wavelength bandλ_(1j) of the optical fiber network NW1 is given by the WDM signal lightillustrated in FIG. 22 and the wavelength of the pump light of the phaseconjugator PC11 is λ_(P), then the wavelength band λ_(2j) of the opticalfiber network NW2 is given by the band of the converted light.

[0218] According such a system construction as described above, sincecompensation for the waveform distortion and the wavelength conversionfunction by a phase conjugator are utilized effectively, implementationof a long-haul large-capacity system which is high in flexibility ispossible. Further, application to transmission between such networks isparticularly important in the following points recently:

[0219] (1) achievement in broader band of an optical amplifier; and

[0220] (2) achievement in variety of the dispersion of an optical fiberused as a transmission line.

[0221] Of the two points, (1) relates to recent achievement in broaderband of an EDFA (erbium-doped fiber amplifier), and (2) relates toachievement in higher speed of a transmission signal and dispersioncontrol for performing WDM transmission. Recently, an EDFA which hassuch a broad band as exceeds 50 nm and is superior in flatness of thegain directed to WDM has been developed. It is estimated that, infuture, the band is further expanded and an EDFA of a broad band ofapproximately 60 to 80 nm is developed. Naturally, while such anincrease of the band of an EDFA contributes to an increase in number ofchannels (transmission capacity) of WDM, introduction of a new conceptin such transmission between networks as illustrated in FIG. 23 isallowed.

[0222] For example, where the wavelength bands of the optical fibernetworks NW1 and NW2 of FIG. 23 are set in such a manner as seen in FIG.24, effective transmission according to the present invention ispossible between the optical fiber networks NW1 and NW2. In FIG. 24,reference numeral 144 denotes a comparatively flat gain band of anoptical amplifier (for example, an EDFA).

[0223] One of reasons why the wavelength bands to be used for theindividual networks are different in this manner resides in that opticalfibers as transmission lines used for the individual networks aredifferent from each other. As optical fibers which have already been putinto practical use, there are a 1.3 μm zero dispersion single mode fiber(so-called standard SMF) and a 1.55 μm dispersion shifted fiber (DSF).Meanwhile, as a result of recent development of EDFAs, the center ofhigh-speed long-haul transmission is moving to the 1.55 μm band. Whilethe standard SMF exhibits a high anomalous dispersion value ofapproximately +16 to +20 ps/nm/km, since the dispersion value of the DSFcan be suppressed to a low value of approximately ±1 to 2 ps/nm/km, theDSF is more advantageous for high-speed long-haul transmission in the1.55 μm band. However, many standard SMFs have been laid already, andalso those networks which must use such standard SMFs as a transmissionline are large in number. For connection from a network of suchconstruction to another network which employs the DSF, waveformconversion into a wavelength band with which an optimum dispersion valueto the DSF is provided is required, and accordingly, the presentinvention is effective for such an instance.

[0224] On the other hand, the present invention is effective also forconnection between networks each of which the DSF is used. The reason isthat, for WDM, a lower dispersion is not necessarily advantageous. ForWDM of a comparatively high speed, in order to secure a required signalto noise ratio (SNR), the power level of each channel must be setconsiderably high. In this instance, if the dispersion of an opticalfiber which is used as a transmission line is small, crosstalk betweenadjacent channels is caused by four wavemixing, and the transmissioncharacteristic is deteriorated. In order to eliminate this influence,recently a fiber (Nonzero dispersion-shifted fiber) having acomparatively large dispersion whose zero dispersion wavelength isshifted by a large amount from the signal band is sometimes used. As thevariety of optical fibers to be used for a transmission line has becomeabundant in this manner, the network constructions in various wavelengthbands are possible, and in connection between such networks, suchwavelength conversion and phase conjugate conversion of a broad band asin the present invention are effective.

[0225] Recently, while also the variety of EDFAs has become abundanttogether with optical fibers, the most popular EDFA is of the type whichhas a gain peak in the 1.53 μm band or the 1.55 μm band. Of the twobands, the former is called blue band and the latter is called red band.

[0226]FIG. 25 is a view illustrating another setting example of awavelength band in FIG. 23. Here, the wavelength band of the opticalfiber network NW1 is included in the red band of the EDFA denoted atreference numeral 146 while the wavelength band of the optical fibernetwork NW2 is included in the blue band of the EDFA denoted atreference numeral 148. According to such setting, where the opticalfiber transmission line 140 or each network includes an EDFA of thein-line type, red band and phase conjugate conversion can be performedreadily.

[0227]FIG. 26 is a view illustrating an example of the dispersionarrangement of FIG. 23. Reference symbols D₁ and D₂ (the unit of each ofwhich is ps/nm/km) represent dispersions of the optical fiber networksNW1 and NW2, respectively. In FIG. 26, an example wherein WDM isperformed using a normal dispersion fiber in each network isillustrated. Since the channel arrangement is reversed by wavelengthconversion as seen in FIG. 22, it is estimated that the influences ofdispersions before and after conversion for each channel are differentfrom each other. However, this problem can be solved by making theinfluences of the dispersions upon channels in the proximity of thecenter substantially equal to each other and performing dispersioncompensation in each network. It is to be noted that the dispersion ineach network may be a normal dispersion or an anomalous dispersion.

[0228] As described above, according to the present invention, there isprovided an optical fiber communication system which includes aplurality of optical fiber networks for WDM signal light obtained bywavelength division multiplexing (WDM), a plurality of optical signalshaving wavelengths different from each other, and at least one converterfor coupling the optical fiber networks to each other. Since theconverter performs wavelength conversion and phase conjugate conversionof the plurality of optical signals collectively, construction of along-haul large-capacity system which is high in flexibility isfacilitated.

[0229]FIG. 27 is a view showing an improvement on the phase conjugatorshown in FIG. 6. Here, first and second optical band block filters 152and 154 and an optical band-pass filter 156 are provided additionally.Signal light (an input beam) is supplied to a port 22A of an opticalcoupler 22 through the first optical band block filter 152, and phaseconjugate light generated in the optical fiber 18 which serves as anonlinear optical medium is outputted after it successively passes thesecond optical band block filter 154 and the optical band-pass filter156 in this order. The order in connection of the filters 154 and 156may be reversed.

[0230] Referring to FIG. 28A, characteristics of the filters 152, 154and 156 shown in FIG. 27 are illustrated. In FIG. 28A, the axis ofordinate indicates the transmittance, and the axis of abscissa indicatesthe wavelength. The first optical band block filter 152 has a blockband, as denoted by reference numeral 158, including a wavelength of thewavelength λ_(c) of phase conjugate light generated in the optical fiber18. In particular, the transmittance of the filter 152 in a region inthe proximity of the wavelength λ_(c) is substantially 0%, and thetransmittance of it in the other regions than the region issubstantially 100%. The second optical band block filter 154 has acomparatively narrow block band, as denoted by reference numeral 160,including the wavelength λ_(P) of pump light outputted from the laserdiode 20. In particular, the transmittance of the filter 154 in a regionin the proximity of the wavelength λ_(P) is substantially 0%, and thetransmittance of it in the other regions than the region issubstantially 100%. The optical band-pass filter 156 has a pass-band, asdenoted by reference numeral 162, including a wavelength of thewavelength λ_(c) of phase conjugate light generated in the optical fiber18. In particular, the transmittance of the filter 156 in a region inthe proximity of the wavelength λ_(c) is substantially 100%, and thetransmittance of it in the other regions than the region issubstantially 0%.

[0231] Referring to FIGS. 28B to 28D, there are shown optical spectraobserved at different positions of the phase conjugator of FIG. 27. FIG.28B shows a spectrum of an output of the first optical band block filter152. Here, signal light is given by WDM signal light superposed on ASElight. Since the first optical band block filter 152 is used, as denotedat reference numeral 164, a window in which noise power is very low isformed in the ASE spectrum. FIG. 28C shows a spectrum of an output ofthe optical fiber 18. As a result of nondegenerative four wave mixing inthe optical fiber 18, phase conjugate conversion and wavelengthconversion are performed so that the WDM signal light is converted intoconverted light. The wavelength arrangements of channels between the WDMsignal light and the converted light are symmetrical with respect to thewavelength λ_(P) of the pump light as described hereinabove. Thewavelengths of the channels of the converted light are included in thewindow 164. FIG. 28D shows a spectrum of an output of the opticalband-pass filter 156. Since the second optical band block filter 154 hasa narrow block band, the power of the pump light is suppressedeffectively. Further, since the optical band-pass filter 156 is adopted,ASE light in the proximity of the window 164 is suppressed effectively.

[0232] In the embodiment of FIG. 27, since the optical band block filter154 for removing pump light is provided on the output side of theoptical fiber 18, the influence of the pump light upon the receivingstation or an optical device disposed on the downstream side of theoptical transmission line is reduced, and processing (extraction,amplification and so forth) of phase conjugate light can be performedreadily. For example, where an optical amplifier is provided on thedownstream side of the phase conjugator, if pump light having a highpower is supplied to the optical amplifier, then there is thepossibility that the optical amplifier may become saturated, resultingin failure to obtain a required gain. However, by adopting such aconstruction as shown in FIG. 27, such a problem as just described canbe solved.

[0233] Particularly, in the embodiment of FIG. 27, since the opticalband block filter 154 and the optical band-pass filter 156 are connectedin cascade connection on the output side of the optical fiber 18,suppression of pump light can be performed effectively. Accordingly, thepower of the pump light can be made high to effectively raise theconversion efficiency. For example, where it is taken into considerationthat, if only the optical band-pass filter 156 is provided on the outputside of the optical fiber 18, then the pump light removing capacity maypossibly be low due to the production technique of the optical band-passfilter 156, the combination of the filters 154 and 156 is effective. Inthis sense, the advantage achieved by the embodiment of FIG. 27 thatpump light and/or signal light can be removed effectively is notself-evident from or is non-obvious or critical over the prior art. Thereason why, in the embodiment of FIG. 27, the optical band block filter152 is provided on the input side of the optical fiber 18 is that it isintended to remove in advance ASE noise in the proximity of thewavelength λ_(c) of phase conjugate light to be generated. As a result,deterioration of the signal to noise ratio (SNR) can be prevented. WhileFIG. 27 shows an improvement on the phase conjugator shown in FIG. 6,similar improvement may be made for the phase conjugator shown in FIG.21. In this instance, the first optical band block filter 152 isprovided between the input port 130 and the port 132A of the opticalcoupler 132, and the second optical band block filter 154 and theoptical band-pass filter 156 are provided between the port 134D of thepolarizing beam splitter 134 and the output port 136.

[0234] As described above, according to the present invention, as anapparatus for generating phase conjugate light, a phase conjugator whichexhibits reduced deterioration in SNR and has a reduced influence on thedownstream side is provided. This phase conjugator includes a nonlinearoptical medium, a pump light source, and an optical band block filter.The nonlinear optical medium has a first end and a second end, andsignal light is supplied to the first end. The pump light sourcesupplies pump light from at least one of the first end and the secondend into the nonlinear optical medium. The optical band block filter isoperatively connected to the second end of the nonlinear optical medium.The optical band block filter has a block band including a wavelength ofthe pump light.

[0235] When the present invention is worked, a fiber grating may be usedfor the optical filters. Where the refractive index of an optical medium(for example, glass) is permanently varied by irradiation of light, themedium is called photosensitive. By using this character, the fibergrating can be produced in the core of an optical fiber. Thecharacteristic of such a fiber grating as just mentioned is that itBragg reflects light in a narrow band in the proximity of a resonancewavelength which is determined by the grating pitch and the effectiverefractive index of a fiber mode. The fiber grating can be produced, forexample, by irradiating an excimer laser which is oscillated with awavelength of 248 nm or 193 nm using a phase mask.

[0236] For example, by producing each of the optical band block filters152 and 154 shown in FIG. 27 using a fiber grating, an accurate andnarrow block band can be obtained.

Industrial Applicability of the Invention

[0237] As described above, according to the present invention, sincechromatic dispersion and nonlinearity can be effectively compensated forusing a phase conjugator, provision of a long-haul large-capacityoptical fiber communication system is allowed. Further, provision of aphase conjugator of a broad conversion band and a high conversionefficiency suitable for use with such a system is allowed.

1. An optical fiber communication system, comprising: a first opticalfiber having a first end and a second end which correspond to an inputend and an output end for a signal beam, respectively; a first phaseconjugator operatively connected to said second end for converting thesignal beam into a first phase conjugate beam and outputting the firstphase conjugate beam; a second optical fiber having a third end and afourth end which correspond to an input end and an output end for thefirst phase conjugate beam, respectively; a second phase conjugatoroperatively connected to said fourth end for converting the first phaseconjugate beam into a second phase conjugate beam and outputting thesecond phase conjugate beam; and a third optical fiber having a fifthend and a sixth end which correspond to an input end and an output endfor the second phase conjugate beam; said second optical fibercomprising a first portion located between said third end and a systemmidpoint and a second portion located between said system midpoint andsaid fourth end; the product of the average value of the chromaticdispersion and the length of said first optical fiber beingsubstantially coincident with the product of the average value of thechromatic dispersion and the length of said first portion; the productof the average value of the chromatic dispersion and the length of saidsecond portion being substantially coincident with the product of theaverage value of the chromatic dispersion and the length of said thirdoptical fiber.
 2. An optical fiber communication system according toclaim 1 , wherein the product of the average value of the optical powerand the average value of the nonlinear coefficient in said first opticalfiber and the length of said first optical fiber substantially coincideswith the product of the average value of the optical power and theaverage value of the nonlinear coefficient in said first portion and thelength of said first portion, and the product of the average value ofthe optical power and the average value of the nonlinear coefficient insaid second portion and the length of said second portion substantiallycoincides with the product of the average value of the optical power andthe average value of the nonlinear coefficient in said third opticalfiber and the length of said third optical fiber.
 3. An optical fibercommunication system according to claim 1 , further comprising aplurality of optical amplifiers provided on an optical path whichincludes said first optical fiber, said second optical fiber and saidthird optical fiber.
 4. An optical fiber communication system accordingto claim 3 , wherein the distance between each two adjacent ones of saidplurality of optical amplifiers is shorter than the nonlinear length ofsaid optical path.
 5. An optical fiber communication system according toclaim 1 , wherein said system midpoint is defined as a position at whichthe waveform distortion of the first phase conjugate beam is minimized.6. An optical fiber communication system according to claim 1 , wherein,when said first optical fiber and said first portion are imaginarilydivided into an equal number of intervals, the products of the averagevalues of the chromatic dispersions and the interval lengths of each twocorresponding intervals as counted from said first phase conjugatorsubstantially coincide with each other and the products of the averagevalues of the optical powers, the average values of the nonlinearcoefficients and the interval lengths of the two intervals substantiallycoincide with each other, and, when said second portion and said thirdoptical fiber are imaginarily divided into an equal number of intervals,the products of the average values of the chromatic dispersions and theinterval lengths of each two corresponding intervals as counted fromsaid second phase conjugator substantially coincide with each other andthe products of the average values of the optical powers, the averagevalues of the nonlinear coefficients and the interval lengths of the twointervals substantially coincide with each other.
 7. An optical fibercommunication system according to claim 1 , wherein the ratios betweenthe products of the optical powers and the nonlinear coefficients andthe chromatic dispersions at each two points on said first optical fiberand said first portion at which the accumulated values of the chromaticdispersions from said first phase conjugator are equal to each othersubstantially coincide with each other, and the ratios between theproducts of the optical powers and the nonlinear coefficients and thechromatic dispersions at each two points on said second portion and saidthird optical fiber at which the accumulated values of the chromaticdispersions from said second phase conjugator are equal to each othersubstantially coincide with each other.
 8. An optical fibercommunication system according to claim 1 , wherein the ratios betweenthe products of the optical powers and the nonlinear coefficients andthe chromatic dispersions at each two points on said first optical fiberand said first portion at which the accumulated values of the productsof the optical powers and the nonlinear coefficients from said firstphase conjugator are equal to each other substantially coincide witheach other, and the ratios between the products of the optical powersand the nonlinear coefficients and the chromatic dispersions at each twopoints on said second portion and said third optical fiber at which theaccumulated values of the products of the optical powers and thenonlinear coefficients from said second phase conjugator are equal toeach other substantially coincide with each other.
 9. An optical fibercommunication system according to claim 1 , wherein each of said firstand second phase conjugators comprises a second-order or third-ordernonlinear optical medium operatively connected to said second opticalfiber, and means for pumping said nonlinear optical medium.
 10. Anoptical fiber communication system according to claim 9 , wherein saidmeans for pumping includes means for supplying pump light to saidnonlinear optical medium, and said nonlinear optical medium comprises anoptical fiber having a zero dispersion wavelength substantially equal tothe wavelength of the pump light.
 11. An optical fiber communicationsystem according to claim 1 , further comprising: an optical transmitterfor supplying the signal beam to said first optical fiber; and anoptical receiver for receiving the second phase conjugate beam from saidthird optical fiber.
 12. An optical fiber communication system accordingto claim 11 , wherein said optical transmitter, said first optical fiberand said first phase conjugator are included in a first terminal stationwhile said second phase conjugator, said third optical fiber and saidoptical receiver are included in a second terminal station, and saidsecond optical fiber is laid between said first and second terminalstations.
 13. An optical fiber communication system according to claim 1, further comprising an optical amplifier provided at said input end orsaid output end or intermediately of said second optical fiber foramplifying the first phase conjugate beam, whereby a loss of said secondoptical fiber is compensated for.
 14. An optical fiber communicationsystem according to claim 13 , further comprising an optical band-passfilter provided in the proximity of said system midpoint of said secondoptical fiber and having a pass-band including the wavelength of thefirst phase conjugate beam, whereby noise produced by said opticalamplifier is removed.
 15. An optical fiber communication systemaccording to claim 13 , wherein said optical amplifier comprises aplurality of optical amplifiers.
 16. An optical fiber communicationsystem according to claim 15 , wherein, the distance between each twoadjacent ones of said plurality of optical amplifiers is set shorterthan the nonlinear length of said second optical fiber.
 17. An opticalfiber communication system according to claim 1 , further comprising abranching unit provided in the proximity of said system midpoint of saidsecond optical fiber for branching the first phase conjugate beam into aplurality of branch beams, each of said second portion, said secondphase conjugator and said third optical fiber being provided by a pluralnumber corresponding to the plurality of branch beams, the plurality ofbranch beams being individually supplied to the plurality of secondportions.
 18. An optical fiber communication system according to claim17 , further comprising means operatively connected to said branchingunit for monitoring the transmission characteristic of the first phaseconjugate beam.
 19. An optical fiber communication system according toclaim 1 , wherein said first optical fiber includes adispersion-decreasing fiber having a chromatic dispersion whichdecreases as the power of the signal beam decreases, whereby the ratiobetween the product of the optical power and the nonlinear coefficientand the chromatic dispersion of said first optical fiber issubstantially constant.
 20. An optical fiber communication systemaccording to claim 19 , wherein said dispersion-decreasing fibercomprises a plurality of dispersion shifted fibers having differentchromatic dispersions from each other and connected in cascadeconnection.
 21. An optical fiber communication system according to claim19 , further comprising a plurality of optical amplifiers for making theoptical power in said second optical fiber substantially constant. 22.An optical fiber communication system according to claim 21 , whereinsaid third optical fiber includes a second dispersion-decreasing fiberhaving a chromatic dispersion which decreases as the power of the secondphase conjugate beam decreases, whereby the ratio between the product ofthe optical power and the nonlinear coefficient and the chromaticdispersion of said third optical fiber is substantially constant.
 23. Anoptical fiber communication system according to claim 19 , wherein saiddispersion-decreasing fiber is provided by a plural number, and saidoptical fiber communication system further comprises at east one opticalamplifier provided between the dispersion-decreasing fibers.
 24. Anoptical fiber communication system according to claim 1 , wherein thesignal beam is a WDM signal beam obtained by wavelength divisionmultiplexing (WDM) a plurality of signal beams having differentwavelengths from each other, and said first optical fiber is provided bya plural number corresponding to the plurality of signal beams.
 25. Anoptical fiber communication system according to claim 24 , furthercomprising an optical demultiplexer operatively connected to said secondphase conjugator for demultiplexing the second phase conjugate beam intoa plurality of phase conjugate beams corresponding to the plurality ofsignal beams, said third optical fiber being provided by a plural numbercorresponding to the plurality of phase conjugate beams.
 26. An opticalfiber communication system according to claim 1 , further comprising atleast one dispersion compensator provided at the input end or the outputend or intermediately of at least one of said first, second and thirdoptical fibers for providing a chromatic dispersion of the opposite signto that of the chromatic dispersion of each of said first, second andthird optical fibers.
 27. An optical fiber communication systemaccording to claim 26 , wherein said dispersion compensator comprises adispersion compensation fiber having a chromatic dispersion per unitlength which is higher in absolute value than that of said secondoptical fiber.
 28. An optical fiber communication system according toclaim 26 , wherein each of said first, second and third optical fibersprovides a normal dispersion, and said dispersion compensator comprisesa 1.3 μm zero dispersion fiber.
 29. An optical fiber communicationsystem according to claim 26 , wherein said dispersion compensatorcomprises a fiber grating.
 30. An optical fiber communication systemaccording to claim 1 , further comprising one or more optical units eachincluding optical elements which correspond to said first optical fiber,said first phase conjugator, said second optical fiber, said secondphase conjugator and said third optical fiber, said optical unit orunits being operatively connected to said third optical fiber.
 31. Anoptical fiber communication system, comprising: a first optical fiberhaving a first end and a second end which correspond to an input end andan output end for a signal beam, respectively; a phase conjugatoroperatively connected to said second end for converting the signal beaminto a phase conjugate beam and outputting the phase conjugate beam; asecond optical fiber having a third end and a fourth end whichcorrespond to an input end and an output end for the phase conjugatebeam, respectively; and at least one dispersion compensator provided onan optical path which includes said first optical fiber, said phaseconjugator and said second optical fiber for providing a chromaticdispersion of the opposite sign to that of the chromatic dispersion ofeach of said first and second optical fibers; the product of the averagevalue of the chromatic dispersion and the length of said first opticalfiber being substantially coincident with the product of the averagevalue of the chromatic dispersion and the length of said second opticalfiber.
 32. An optical fiber communication system according to claim 31 ,wherein the product of the average value of the optical power and theaverage value of the nonlinear coefficient in said first optical fiberand the length of said first optical fiber substantially coincides withthe product of the average value of the optical power and the averagevalue of the nonlinear coefficient in said second optical fiber and thelength of said second optical fiber.
 33. An optical fiber communicationsystem according to claim 31 , further comprising a plurality of opticalamplifiers provided on an optical path which includes said first opticalfiber and said second optical fiber.
 34. An optical fiber communicationsystem according to claim 33 , wherein the distance between each twoadjacent ones of said plurality of optical amplifiers is shorter thanthe nonlinear length of said optical path.
 35. An optical fibercommunication system according to claim 31 , wherein, when said firstand second optical fibers are imaginarily divided into an equal numberof intervals, the products of the average values of the chromaticdispersions and the interval lengths of each two corresponding intervalsas counted from said phase conjugator substantially coincide with eachother and the products of the average values of the optical powers, theaverage values of the nonlinear coefficients and the interval lengths ofthe two intervals substantially coincide with each other.
 36. An opticalfiber communication system according to claim 31 , wherein the ratiosbetween the products of the optical powers and the nonlinearcoefficients and the chromatic dispersions at each two points on saidfirst and second optical fibers at which the accumulated values of thechromatic dispersions from said phase conjugator are equal to each othersubstantially coincide with each other.
 37. An optical fibercommunication system according to claim 31 , wherein the ratios betweenthe products of the optical powers and the nonlinear coefficients andthe chromatic dispersions at each two points on said first and secondoptical fibers at which the accumulated values of the products of theoptical powers and the nonlinear coefficients from said phase conjugatorare equal to each other substantially coincide with each other.
 38. Anoptical fiber communication system according to claim 31 , wherein saiddispersion compensator is provided intermediately of said first opticalfiber.
 39. An optical fiber communication system according to claim 31 ,wherein said dispersion compensator is provided intermediately of saidsecond optical fiber.
 40. An optical fiber communication systemaccording to claim 31 , wherein each of said first and second opticalfibers is a single mode fiber.
 41. An optical fiber communication systemaccording to claim 40 , wherein said single mode fiber provides ananomalous dispersion in a wavelength 1.55 μm band.
 42. An optical fibercommunication system according to claim 31 , wherein said dispersioncompensator comprises a fiber grating.
 43. An optical fibercommunication system according to claim 31 , wherein the value of thechromatic dispersion of said dispersion compensator is set so that thewaveform distortion of the phase conjugate beam at said fourth end ofsaid second optical fiber may be minimized.
 44. An optical fibercommunication system according to claim 31 , wherein each of said firstand second optical fibers provides a normal dispersion, and saiddispersion compensator comprises a 1.3 μm zero dispersion fiber.
 45. Anoptical fiber communication system, comprising: a first optical fiberhaving a first end and a second end which correspond to an input end andan output end for a signal beam, respectively; a phase conjugatoroperatively connected to said second end for converting the signal beaminto a phase conjugate beam and outputting the phase conjugate beam; anda second optical fiber having a third end and a fourth end whichcorrespond to an input end and an output end for the phase conjugatebeam, respectively; the product of the average value of the chromaticdispersion and the length of said first optical fiber beingsubstantially coincident with the product of the average value of thechromatic dispersion and the length of said second optical fiber; thesignal beam being a wavelength division multiplexed signal beamincluding a plurality of signal beams having wavelengths different fromeach other; both of the second-order dispersions (dispersion gradients)of said first and second optical fibers being substantially equal tozero or having signs opposite to each other.
 46. An optical fibercommunication system according to claim 45 , wherein the product of theaverage value of the optical power and the average value of thenonlinear coefficient in said first optical fiber and the length of saidfirst optical fiber substantially coincides with the product of theaverage value of the optical power and the average value of thenonlinear coefficient in said second optical fiber and the length ofsaid second optical fiber.
 47. An optical fiber communication systemaccording to claim 45 , further comprising a plurality of opticalamplifiers provided on an optical path which includes said first opticalfiber and said second optical fiber.
 48. An optical fiber communicationsystem according to claim 47 , wherein the distance between each twoadjacent ones of said plurality of optical amplifiers is shorter thanthe nonlinear length of said optical path.
 49. An optical fibercommunication system according to claim 45 , wherein, when said firstand second optical fibers are imaginarily divided into an equal numberof intervals, the products of the average values of the chromaticdispersions and the interval lengths of each two corresponding intervalsas counted from said phase conjugator substantially coincide with eachother and the products of the average values of the optical powers, theaverage values of the nonlinear coefficients and the interval lengths ofthe two intervals substantially coincide with each other.
 50. An opticalfiber communication system according to claim 45 , wherein, the ratiosbetween the products of the optical powers and the nonlinearcoefficients and the chromatic dispersions at each two points on saidfirst and second optical fibers at which the accumulated values of thechromatic dispersions from said phase conjugator are equal to each othersubstantially coincide with each other.
 51. An optical fibercommunication system according to claim 45 , wherein, the ratios betweenthe products of the optical powers and the nonlinear coefficients andthe chromatic dispersions at each two points on said first and secondoptical fibers at which the accumulated values of the products of theoptical powers and the nonlinear coefficients from said phase conjugatorare equal to each other substantially coincide with each other.
 52. Amethod for producing an apparatus which has a nonlinear optical mediumfor generating phase conjugate light, said method comprising the stepsof: (a) cutting an optical fiber into a plurality of intervals; and (b)re-arranging and joining together the plurality of intervals so that aconversion band in nondegenerative four wave mixing in which saidnonlinear optical medium is used may be maximized to obtain saidnonlinear optical medium.
 53. A method according to claim 52 , whereinthe step (b) includes a step of measuring the dispersion value of eachof the plurality of intervals, and the plurality of intervals arere-arranged such that those of the intervals which have comparativelylow dispersion values are disposed adjacent an input end of saidnonlinear optical medium when pump light is inputted to said nonlinearoptical medium.
 54. A method according to claim 52 , wherein at leastsome of the plurality of intervals are joined together such that thepositive and negative signs of the dispersion values appear alternately.55. An apparatus produced by a method according to claim 52 .
 56. Amethod for producing an apparatus which has a nonlinear optical mediumfor generating phase conjugate light, said method comprising the stepsof: (a) cutting an optical fiber into a plurality of intervals; (b)measuring the dispersion value of each of the plurality of intervals;and (c) selecting and joining together only those of the intervals whichhave dispersion values sufficiently low to obtain a required conversionband in nondegenerative four wave mixing in which said nonlinear opticalmedium is used to obtain said nonlinear optical medium.
 57. An apparatusproduced by a method according to claim 56 .
 58. A method for producingan apparatus which has a nonlinear optical medium for generating phaseconjugate light, said method comprising the steps of: (a) measuring thedeviation of the zero dispersion wavelength of an optical fiber; (b)cutting, when the deviation exceeds a range determined in advance, theoptical fiber so that the cut fibers may have deviations in zerodispersion wavelength which remain within the range determined inadvance; and (c) selecting and joining together the optical fiber or thecut fibers each having a zero dispersion wavelength substantially equalto the wavelength of pump light to obtain said nonlinear optical medium.59. An apparatus produced by a method according to claim 58 .
 60. Anoptical fiber communication system, comprising: a plurality of opticalfiber networks for WDM signal light obtained by wavelength divisionmultiplexing (WDM) a plurality of optical signals having wavelengthsdifferent from each other; and at least one converter for coupling saidplurality of optical fiber networks to each other; said converterincluding means for performing wavelength conversion and phase conjugateconversion of the plurality of optical signals collectively.
 61. Anoptical fiber communication system according to claim 60 , furthercomprising an optical adding/dropping apparatus for the optical signals,said optical adding/dropping apparatus being provided at a position atwhich the waveform distortion of the optical signals is minimized. 62.An optical fiber communication system according to claim 60 , whereinthe wavelength conversion is performed between arbitrary bands includedin a gain band provided by an erbium-doped fiber amplifier.
 63. Anoptical fiber communication system according to claim 62 , wherein thearbitrary bands are a 1.55 μm band and a 1.53 μm band.
 64. An apparatusfor generating phase conjugate light, comprising: a polarization beamsplitter having first to fourth ports, said first and second ports andsaid third and fourth ports being coupled to each other by a firstpolarization plane, said first and third ports and said second andfourth ports being coupled to each other by a second polarization planeperpendicular to the first polarization plane; a nonlinear opticalmedium operatively connected between said second and third ports; a pumplight source for outputting pump light; and coupling means for supplyingsignal light and the pump light to said first port of said polarizationbeam splitter; said coupling means including an optical coupler havingfirst and second input ports for receiving the signal light and the pumplight, respectively, and an output port, and a polarization maintainingfiber operatively connected between said output port and said firstport.
 65. An apparatus according to claim 64 , wherein said polarizationmaintaining fiber has a principal axis set so that the polarizationplane of the pump light at said first port is inclined substantially by45 degrees with respect to the first and second polarization planes. 66.An apparatus according to claim 65 , wherein said polarizationmaintaining fiber comprises first and second polarization maintainingfibers connected to each other by splicing, and said first and secondpolarization maintaining fibers have first and second principal axeswhich extend perpendicularly to each other.
 67. An apparatus forgenerating phase conjugate light, comprising: a nonlinear optical mediumhaving a first end and a second end, said first end being supplied withsignal light; a pump light source for supplying pump light from at leastone of said first end and said second end into said nonlinear opticalmedium; and an optical band block filter operatively connected to saidsecond end of said nonlinear optical medium and having a block bandincluding the wavelength of the pump light.
 68. An apparatus accordingto claim 67 , further comprising an optical band-pass filter operativelyconnected to said optical band block filter and having a pass-bandincluding the wavelength of phase conjugate beam generated in saidnonlinear optical medium.
 69. An apparatus according to claim 67 ,further comprising a second optical band block filter operativelyconnected to said first end of said nonlinear optical medium and havinga block band including the wavelength of phase conjugate beam generatedin said nonlinear optical medium.
 70. An apparatus according to claim 67, wherein said nonlinear optical medium includes an optical fiber. 71.An apparatus according to claim 70 , wherein said optical fiber has azero dispersion wavelength substantially equal to the wavelength of thepump light.
 72. An apparatus according to claim 70 , wherein saidoptical fiber has a nonlinear coefficient sufficiently high to make thelength of said optical fiber short to such a degree that said opticalfiber has a polarization plane maintaining capacity.
 73. An apparatusaccording to claim 72 , wherein said optical fiber includes a core dopedwith GeO₂ and a clad doped with fluorine.
 74. An apparatus according toclaim 72 , wherein said optical fiber comprises a single mode fiber, andsaid single mode fiber has a mode field diameter smaller than the modefield diameter of a single mode fiber which is used as a transmissionline.
 75. An optical transmission apparatus connected to an input end ofan optical fiber transmission line, comprising: a first optical fiberhaving an input end and an output end and having a chromatic dispersionamount substantially equal to a chromatic dispersion amount between amidpoint and said input end of said optical fiber transmission line;optical signal outputting means for inputting an optical signal to saidinput end of said first optical fiber; and a phase conjugator forconverting the optical signal from said output end of said first opticalfiber into phase conjugate light which has a phase conjugaterelationship with the optical signal and inputting the phase conjugatelight to said input end of said optical transmission line.
 76. Anoptical reception apparatus connected to an output end of an opticalfiber transmission line which transmits an optical signal, comprising: asecond optical fiber having an input end and an output end and having achromatic dispersion amount substantially equal to a chromaticdispersion amount between a midpoint and said output end of said opticalfiber transmission line; a phase conjugator for converting the opticalsignal received from said optical fiber transmission line into phaseconjugate light which has a phase conjugate relationship with theoptical signal and inputting the phase conjugate light into said inputend of said second optical fiber; and a receiver for receiving the phaseconjugate light from said output end of said second optical fiber. 77.An optical communication system including an optical transmissionapparatus connected to an input end of an optical fiber transmissionline, and an optical reception apparatus connected to an output end ofsaid optical fiber transmission line, said optical transmissionapparatus comprising: a first optical fiber having an input end and anoutput end and having a chromatic dispersion amount substantially equalto a chromatic dispersion amount between a midpoint and said input endof said optical fiber transmission line; optical signal outputting meansfor inputting an optical signal to said input end of said first opticalfiber; and a first phase conjugator for converting the optical signalfrom said output end of said first optical fiber into first phaseconjugate light which has a phase conjugate relationship with theoptical signal and inputting the first phase conjugate light to saidinput end of said optical transmission line; said optical receptionapparatus comprising: a second optical fiber having an input end and anoutput end and having a chromatic dispersion value substantially equalto a chromatic dispersion amount between said midpoint and said outputend of said optical fiber transmission line; a second phase conjugatorfor converting the first phase conjugate light received from saidoptical fiber transmission line into second phase conjugate light whichhas a phase conjugate relationship with the phase conjugate light andinputting the second phase conjugate light into said input end of saidsecond optical fiber; and a receiver for receiving the second phaseconjugate light from said output end of said second optical fiber.